Trivial fix on migration doc
Trivial fix on migration doc

This patch does the following:
1. Mention that the current doc is only relavant to cold-migration.
2. Additional live-migration reference is given.
3. The inappropriate --live flag in the example is removed.
4. Policy violation message is updated.
5. Replace a nova command with openstack commmand

Change-Id: Idaa7915ea47d11e30da3f12318082a10a4e73b3b

rahulunair/nova,openstack/nova,gooddata/openstack-nova,mahak/nova,klmitch/nova,gooddata/openstack-nova,rahulunair/nova,gooddata/openstack-nova,mahak/nova,mikalstill/nova,gooddata/openstack-nova,openstack/nova,klmitch/nova,rahulunair/nova,mahak/nova,openstack/nova,klmitch/nova,klmitch/nova,mikalstill/nova,mikalstill/nova