Remove ovs-vsctl call from OVSInterfaceDriver

ovs_lib module is responsible for the interaction with OVS but
OVSInterfaceDriver._ovs_add_port() method calls directly
ovs-vsctl which implies inconsistencies: ovs_lib calls ovs-vsctl
with a timeout but not OVSInterfaceDriver._ovs_add_port().

This change moves ovs-vsctl call in OVSInterfaceDriver._ovs_add_port
to ovs_lib.

Closes-Bug: #1396489
Change-Id: I4d03f8beef2c5f2366ad2d1cf5f0b18b4f2857cd
Merge "Remove ovs-vsctl call from OVSInterfaceDriver"