Change name of SecretMetadatum table to SecretStoreMetadatum

Instead of holding all the metadata attributes for Secrets, this table will
hold only the metadata generated by the SecretStore.  For the SecretStore,
see https://review.openstack.org/#/c/94710/1 .

blueprint add-metadata-to-secrets
Change-Id: I83115671f62e7acfd00bdfec0d4ede653284ac85
Merge "Change name of SecretMetadatum table to SecretStoreMetadatum"