We want external plugins to be able to make use of the core testing utils.
This commit exposes the basic utilities which are currently in use in
bst-external plugins. If necessary, more utilities could be exposed in the
future.

Moves the following files from tests/testutils/ to
buildstream/plugintestingutils/:
o runcli.py
o integration.py

As part of this, this commit makes the following changes to runcli.py
and integration.py:
o runcli.py: Fix linting errors
o runcli.py: Add user facing documentation
o Integration.py: Add user facing documentation

They are too specific to be included by default

Recommendation is if you are building in Linux is to use the
ones begin used in the freedesktop-sdk project, for example

See #645

Also update tests to be consistent with this

In element, platform asks for host os and architecture to default to when
SandboxConfig is initialised.
This changes element cache keys so those have been updated in the tests.

Instead of tag information being fetched which can change with time,
they are tracked and saved in the projects.refs/.bst. Then we re-tag
automatically the closest tag so that `git describe` works and is
reproducible.

This new feature is opt-in with the new `track-tags` configuration,
and must be used to fix modules which are broken by our new policy
of omitting the `.git/` repository when staging git sources.

This fixes issue #487

Fixes #645.

The current directory isn't always in the python module search path,
so we have to ensure it is for the script to work.

Strictly speaking, the user may already have a modified PYTHONPATH
at which point PYTHONPATH=".${PYTHONPATH+:$PYTHONPATH}" is necessary,
but it's probably premature to overcomplicate the documentation like that
before we discover it's a problem.

Since we now set PWD in the environment of builds
existing builds may behave differently so must cache differently now.