Drop service* tables only if they exist

Tables servicedefinitions and servicetypes are not used by neutron but
created using db migration tool. If quantum-server in grizzly was
started without using migration tool then tables mentioned above are not
created. During migration from grizzly to havana alembic attempts to
drop these unused tables. This patch uses "IF EXISTS" clause when
dropping mentioned tables.

Closes-bug: #1273355
Change-Id: I33fcb3e1dc96ce37ec9c00987cb5a3a953ca691d
Reference new get_engine() method from wsgi.py

wsgi.py referenced get_engine in neutron.openstack.common.db.sqlalchemy.session.
This method was removed from that file in commit 53609f29f3c8fcadc545afb891189253c07b33c3
change-id I0e1d86878d3eb924b01e04dced0f90b4e57757d8. This patch references the
replacement method added to neutron.db.api.

Closes-Bug: #1314850
Change-Id: Ifd4db5f49e2aaebc67eccb8bdcef8eea54983112
Remove duplicate module-rgx line in .pylintrc

The correct neutron configuration will no longer be overwritten by
the old quantum configuration.

Change-Id: I4923ad4e35a5053966edb307587c72c0b684d149
Closes-Bug: 1316334
Merge "Reference new get_engine() method from wsgi.py"
Merge "Remove duplicate module-rgx line in .pylintrc"
Merge "Drop service* tables only if they exist"