Makes sure that `cache-junction-elements` and `ignore-junction-remotes`
work together correctly, and that setting `ignore-junction-remotes`
doesn't just remove all remote caches.

As discussed over the mailing list, reformat code using Black. This is a
one-off change to reformat all our codebase. Moving forward, we
shouldn't expect such blanket reformats. Rather, we expect each change
to already comply with the Black formatting style.

'strip_node_info' would be useful for multiple plugins. We should
therefore allow users to use it.

This option allows us to completely ignore remotes defined in the
subproject. As a user, we may not wish to interact with subproject
remotes as we may not be able to trust them, for example.

An appropriate test and some documentation has also been added.

The 'cache-junction-elements' configuration option for junction's
is a boolean which enables elements from within the junction to
interact with the parent project's remote(s).

A test has been added to enforce this behaviour and the
BST_FORMAT_VERSION has been bumped.

Originally, the intended default behaviour of BuildStream was to
only cache project elements in project specific remotes.

This behaviour was broken by: 24c0de16faec2b8b9bd6a03504ce951dc49afbe2.
Introduced in !1113.

This patch reverts the behaviour and asserts that junctioned elements
are not pushed to their parent's project-specific remote.

Now that both are equivalent, we can skip the sanitization part before
the yaml call.

- Implement __setitem__ on 'MappingNode'
- Implement __setitem__ on 'SequenceNode'
- Adapt all call sites to use the new calling way.

Assert that junction elements are pushed to a parent project's
cache. This is the current default behaviour in BuildStream.