Improve neutron CLI existing tests

Objective of this patch to parse the output of read only neutron CLI's, 
to validate the output and improve more output verification.
It is improving the code of all below read only neutron CLI's sub-commands:
1. net-list
2. net-external-list
3. port-list
4. router-list
5. security-group-rule-list
6. subnet-list

Partially implements: blueprint missing-cli-tests-in-tempest

Task is managed using etherpad :
https://etherpad.openstack.org/p/missing-cli-tests-in-tempest)

Change-Id: I9b8c9cd0ab115a08519c0d2474c41836a984cce2
1 file changed