Instance fails to start when using a previously used port

Description of problem:

An instance fails to start when it's attached with a previously port.

Version-Release number of selected component (if applicable):
python-networking-odl-11.0.0-0.20170721155128.a047729.el7ost.noarch
opendaylight-6.1.0-2.el7ost.noarch

How reproducible:

Steps to Reproduce:
1. Create a port
2. Launch an instance with the above port
3. Terminate the instance
4. Launch an instance with the above port
5. Verify that the instance is in 'spawning' state and then Error state