resolve table names using MetaData.schema default in declarative registry

resolve table names using MetaData.schema default in declarative registry

Also resolved class-level MetaData not being consulted by the
declarative class registry when resolving string-based table
references.  The registry now uses the same metadata resolution
logic as table creation, checking for a class-specific ``metadata``
attribute before falling back to ``registry.metadata``.  The
``_metadata_for_cls`` function was factored into ``orm/util.py``
for shared use by both ``decl_base.py`` and ``clsregistry.py``.

Fixes: #8068
Fixes: #13291
Change-Id: Ib846be0267f9295a5fee945dc6cf0a72c237bd2c