Add Neutron port check for the creation of multiple instances

Exception is thrown if multiple instances are launched with a single
Neutron port. Remove one duplicated test case
"test_create_instance_with_security_group_enabled".

Fixed Bug 1193434.

Change-Id: Ie5d426db0e0785043833e4e7f9a16c9591bee743
Merge "Add Neutron port check for the creation of multiple instances"