system_resolver is created based on loaded configuration, which could
avoid calls like `GenericResolver.create(conf)` repeatedly in the code.
However, if some cases need to create a specific resolver explicitly,
`GenericResolver.create` could be called with addition argument, for
example db or mbs is passed to argument backend in tests.
Signed-off-by: Chenxiong Qi <cqi@redhat.com>
We can't rely on matching name-stream to distinguish which module is
currently being built as modules can buildrequire older versions of
the same module-stream.
Fixes#954
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.
