Only set X-Auth-User, X-Auth-Key on stack create/update.

All heat operations can be performed with only a token.

User credentials are required for stack create/update as they are
stored for later stack operations.

This change prevents credentials unnecessarily being sent over the wire.

Change-Id: I5ad18067c5db8ada9994d131e98f27af70a7ca06
Merge "Only set X-Auth-User, X-Auth-Key on stack create/update."