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
Merge "Improve neutron CLI existing tests"