Add _alembic_quote method to format_constraint()

Add _alembic_quote method to format_constraint()

Alembic needs a portable way of getting at the name of an
index without quoting being applied.   As we would like the
indexes created by the Column index=True flag to support
deferred index names, supply a function that delivers this
for Alembic without it having to dig too deeply into the
internals.    the _alembic_quote flag may be made public
at a later time, however as we've been through many quoting
flags that are difficult to get rid of, try to be conservative
to start.

Change-Id: I184adaeae26c2e75093aaea5ebe01a3815cadb08