Fixed test test_update_port_security_off_address_pairs

This test checks that port-update fails with 409 return code when
port-security is turned off on a port which contains address-pairs.
The test had a typo ('security-groups' -> 'security-group') which would
have cause it to produce false-positive.
This change rewrites the test and eliminates the possibility of this
false-positive to occurs.

Change-Id: Ib54ad089872f28eb28a3d4d050ba0ee8fdc2bfe2
Closes-Bug: #1400725
Signed-off-by: Roey Chen <roeyc@vmware.com>
Merge "Fixed test test_update_port_security_off_address_pairs"