Add short form aliases to commands

[Sumit Naiksatam]

For example:

policy-target-group-list could be ptg-list

In effect, the following can acronymized:

external-policy-* -> ep-*
external-segment-* -> es-*
group-* -> g-*
l2policy-* -> l2p-*
l3policy-* -> l3p-*
nat-pool-* -> np-*
network-service-policy-* -> nsp-*
policy-action-* -> pa-*
policy-classifier-* -> pc-*
policy-rule-* -> pr-*
policy-rule-set-* -> prs-*
policy-target-* -> pt-*
policy-target-group-* -> ptg-*
servicechain-instance-* -> sci-*
servicechain-node-* -> scn-*
servicechain-spec-* -> scs-*

[Sumit Naiksatam (snaiksat)]

Fixed by: https://review.openstack.org/#/c/234156/