Don't try to delete trust if trustor not found
Don't try to delete trust if trustor not found

Keystone deletes the trust when delete the trustor, so
don't try to delete the trust again in heat delete stack
with another user in same project.

Change-Id: I2376f2702672d28b24de5ac4ad0bb81fd7171abe
Closes-Bug: #1634362

openstack/heat,noironetworks/heat,noironetworks/heat,openstack/heat