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
Merge "Reference new get_engine() method from wsgi.py"