Instead of using strings (which can be easy to
mistype and get wrong), provide a set of constants
that can be used to attach and use these keys in
flows and at compilation.

This also helps make it more clear what the keys
do and where they are used.

Change-Id: I5283b27617961136a4582bbcfce4617f05e8dd1d

In order to make it possible to have a symbol
tree we need to relax the constraints that are
being imposed by the graph flow.

Part of blueprint taskflow-improved-scoping

Change-Id: I2e14de2131c3ba4e3e4eb3108477583d0f02dae2

To complement the future changes in patterns we also want
to allow the execution of patterns to be affected in a similar
manner so that symbol lookup is no longer as confined as it was.

This change adds in the following:

- Symbol lookup by walking through an atoms contained scope/s.
- Better error messaging when symbols are not found.
- Adjusted & new tests (existing ones work).
- Better logging of the symbol lookup mechanism (helpful
  during debugging, although it is very verbose...)

Part of blueprint taskflow-improved-scoping

Change-Id: Id921a4abd9bf2b7b5c5a762337f8e90e8f1fe194

Instead of blindly assuming all the symbols that
are provided automatically work for all flows even
if the flow has ordering constraints we should set
the base flow class requires property to be abstract
and provide flow specific properties that can do the
appropriate analysis to determine what the flows
unsatisfied symbol requirements actually are.

Part of blueprint taskflow-improved-scoping

Change-Id: Ie149c05b3305c5bfff9d9f2c05e7e064c3a6d0c7

Update hacking to the new requirements version and
fix about half of the new reported issues. The other
hacking issues are for now ignored until fixed by
adjusting our tox.ini file.

This commit fixes the following new hacking errors:

H405 - multi line docstring summary not separated
       with an empty line
E265 - block comment should start with '# '
F402 - import 'endpoint' from line 21 shadowed by
       loop variable

Change-Id: I6bae61591fb988cc17fa79e21cb5f1508d22781c

Code that calculates provides and requires for flow is almost
identical for all patterns, so this change makes it completely
identical and puts it to the base class. Other patterns are
still allowed to override these properties for sake of customization
or optimization.

Change-Id: I6e875e863047b5287ec727fc9a491f252f144ecf

Most of the utility graph functions we have can
be connected to a directed graph class that itself
derives (and adds on to) the networkx base class.

Doing this allows for functionality that isn't exposed
in networkx to be exposed in our subclass (which is a
useful pattern to have).

It also makes it possible (if ever needed) to replace
the networkx usage in taskflow with something else if
this ever becomes a major request.

Change-Id: I0a825d5637236d7b5dbdbda0d426adb0183d5ba3

Group the exceptions into the following groups
  * Storage
  * Jobs
  * Execution
  * Other (wrapped failure here)

This grouping makes it easier to understand where
one type of exception should be used vs using another
type of exception.

Backwards incompatible changes:
  * StorageError -> StorageFailure
  * AlreadyExists -> Duplicate
  * WrappedFailure now inherits from Exception
    and not directly from TaskFlowException since it
    wraps arbitrary other exceptions and is not
    specific to taskflow.

Cleanups:
  * JobNotFound -> NotFound
  * EmptyFlow -> Empty
  * JobAlreadyExists -> AlreadyExists
  * InvariantViolation (X)
  * ConnectionFailure (X)

Change-Id: I0e1e81b513fbbc7adb8bfaa1244993e345ab70d3

This commit adds unit tests that check graph flow methods without
executing or flattening it. Now-redundant tests from other test
suites are deleted.

Change-Id: I8dafe0f9b295428831eddb3f9fd48f042d2f1ffc

In addition to iteration over its children (atoms or subflows) each
pattern now provides iter_links() method that iterates over
dependency links. This allows engines to treat all patterns
in the same way instead knowing what structure each pattern expresses.

Change-Id: I52cb5b0b501eefc8eb56a9ef5303aeb318013e11