decouple index name generation from sqlalchemy version

In commit 0.7.2-16-gc670d1d the _index_identifier() implementation
was copied from sqlalchemy, as that function was renamed
in sqlalchemy 0.8.  Instead handle call the renamed function
when appropriate, to decouple ourselves from the sqlalchemy
implementation.

Change-Id: I97b22c20d96758fc5b6bd55318218edb26c5b5d0