Fix sqlalchemy error regarding identifier length.

sqlalchemy was complaining:

sqlalchemy.exc.IdentifierError: Identifier
'ps_contacts_qualifyfreq_exptime' exceeds maximum length of 30
characters

This fixes the problem by changing the index name to be
"ps_contacts_qualifyfreq_exp" instead.

ASTERISK-26227 #close
Reported by Mark Michelson

Change-Id: I0ed784f87504be2a59ee8d3242ef6f625d5ed1a9