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
    some time.
    
    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.