The check-glanceclient-dsvm-functional gate job was failing due to the
functional tests not picking up valid OpenStack credentials.
Update how we aquire credentials -- fixes side effect of how tox 2.0
handles environment variables.
Change-Id: Ia665dc9673d09421508476d05039d2da8a5e1a29
Closes-bug: 1455102
9f5c58193b