Improve the format hint of -Pf option
Improve the format hint of -Pf option

Change the format hint of -Pf option for stack-create/stack-preview
/stack-update commands, from <KEY=VALUE> to <KEY=FILE>.

Closes-Bug: #1486855
Change-Id: Ib6117325d1b582710bd6b9cddfdbc5e9971b3707

ecerulm/python-heatclient,ecerulm/python-heatclient,openstack/python-heatclient