This reverts commit 8173040ea6. Since it didn't
provide any real benefit after talking with sgallagh, it seems like we'd better
off not introduce more risk to the next deployment.
MBS has been using the libmodulemd API incorrectly by assuming that
methods like `get_rpm_components` return the actual object in memory
and not a copy. This has been true but wasn't something sgallagh
intended. We found this out after he had me test his new version of
libmodulemd. This PR removes those assumptions.
1. Changed ModuleBuild's context property to db column, it's
non-nullable and default value is '00000000' to keep it consistent
with previous behaviour.
2. Changed ModuleBuild.contexts_from_mmd to return a tuple of
(ref_build_context, build_context, runtime_context, context).
3. Updated tests affected by this change.
