The document.py `is_control` method incorrectly checks if a document
is a Control document. Per the documentation [0], Control documents
have `metadata.schema` of `metadata/Control/v1`. This commit updates
the `is_control` method to correctly check for Control documents.
[0] 1d4cc81dfa/doc/source/users/document-types.rst (control-documents)
Change-Id: I60ca8f31a61987b4e756784fce0f5a751639ae9e