Skip a test for nicira service plugin

test_router_add_interface_subnet_with_port_from_other_tenant
is causing intermittent failures in unit tests because of
issues related with sql session autoflush.

This patch skips this test, since it is already covered
by another test case in the same module. This should prevent
job failures while the relevant bug is addressed.

Change-Id: Iad33dada61d17fad14e569e241f4079465b5f792
Related-Bug: 1280035
Merge "Skip a test for nicira service plugin"