Removed session from fixed_ip_*() functions.

Removed session objects from public fixed_ip_*() functions
in nova/db/sqlalchemy/api module. Minor code refactiring.

blueprint db-session-cleanup

Change-Id: Ic2a96270745799496e365aee0380ac079e1edeff
Merge "Removed session from fixed_ip_*() functions."
Merge remote-tracking branch 'm_src_repo/citrix-fixes' into build-temp