Fix JujuEpc prepare() and deploy_orchestrator()

It bypasses a known unfixed Juju bug [1] by creating a second
'_member_' user [1]. An 'admin' user is still required to create
flavors and by snaps when listing virtual machines.
It highlights a possible bug in user_creator.get_os_creds().

It partially rewrites the testcase by cleaning instance attributes and
by taking several os.system() returns into account. It sets the new
password automatically generated [2] and starts switching from
openstack utils to snaps in vEPC.

This patchset must be completed by unit tests and by another patch
fully switching to snaps in the last methods.

It should be noted that the testcase sets 8.8.8.8 as dns server which
could raise side effects.

[1] https://bugs.launchpad.net/juju/+bug/1710848
[2] https://gerrit.opnfv.org/gerrit/#/c/50747/

Change-Id: Id18b4b185aa1825196e91f6692338169f51faea8
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>