use openrc credentials to verify swift installation

Don't use the keystone admin token to authenticate, but a real user
instead (using the openrc.sh config file). This simplifies the swift
commands, and makes the verification more consistent with the
verification of other services.

Change-Id: I6a2512cb6cbb2942c9e8673f3f7d960f2f740e8e
backport: none
(cherry picked from commit 31f384e6260f3bfe8864909537d982f0033c511d)