Allow setting context in the imported MMD file.
We always set the "context" to DEFAULT_MODULE_CONTEXT from the historical
reasons - the context was not stored in the database before and we stored
just the build_context/runtime_context. But this is no longer true for
In this commit, the context is respected and stored in the database when
importing module using the `import_mmd` method. If the context is not set
in the imported MMD, the DEFAULT_MODULE_CONTEXT is used.