Adds override for the overcloud node user in upgrade-non-controller
Adds override for the overcloud node user in upgrade-non-controller

The upgrade-non-controller.sh script currently assumed there is a
heat-admin user on the overcloud nodes. This may not be the case in
some environments. Allow override by export UPGRADE_NODE_USER=foo
before running the upgrade-non-controller.sh script. The user
must have passwordless sudo priviliges otherwise the upgrade script
will fail.

Change-Id: I33026dff7f89032c43f31eaf03fd15fcb09d0050

openstack/tripleo-common,openstack/tripleo-common