remove explicit include of the ovs plugin

remove explicit include of the ovs plugin

On installing only neutron-linuxbridge-agent package, the
dhcp cannot start successfully because of the imports from
ovs plugin. This change removes the explicit include of the
ovs plugin from ovs_lib.py. INVALID_OFPORT has been moved to
ovs_lib.py while VXLAN_UDP_PORT has moved to
plugins/common/constants.py. The imports for these 2 constants
in files which uses it has been corrected to new location.

Closes-Bug: #1271449
Change-Id: I6559cb43d1b10b4f926c453a103b12017b59f259