When neither --tar nor --directory are provided,
bst artifact checkout will no longer error out.
Defaults to a suitable directory, consistent with bst workspace open.
Test for new feature has been added

Add max-jobs configuration and command line option

Closes #1033

See merge request BuildStream/buildstream!1473

This tests that the resolution of the `max-jobs` automatic variable
is properly controlled by the new user configuration and command
line option, including the default automatic '0' value.

Regression test for #1033

Only use automatic max-jobs by default if the user has not
requested a specific value for max-jobs.

This is a part of #1033

While this is currently only relevant for `bst build`, it is
a current implementation detail that user configuration which
gets overridden by command line options must use toplevel
options.

This patch allows invocations such as the following to override
the max-jobs:

    bst --max-jobs 16 build target.bst

This also updates the completions test to expect the new toplevel option.

This is a part of #1033

This loads the configuration of the max-jobs variable
from the user configuration, where the default is 0
(meaning use the maximum number of cores with a limit of 8).

This is a part of #1033

Rewrite of the Node API

See merge request BuildStream/buildstream!1472

We need to update every place where we were passing a yaml 'dict' to
now pass a 'MappingNode'