tests: Pick up rotated XSRF token after login

At some point, Django started rotating XSRF tokens after login.  If we
don't pick up the rotated token, the tests fail with a 403 error and the
message "CSRF verification failed. Request aborted."

Signed-off-by: Tim Serong <tserong@suse.com>
tests: don't try to accept existing minions

When running against an external calamari instance, successive tests
re-authorize existing minions, which fails with:

400: {"detail": {"status": ["Transition accepted->accepted is invalid"]}}

To avoid this, we're now checking if minions are already accepted first.

Signed-off-by: Tim Serong <tserong@suse.com>
tests: correctly catch stderr output when sshing to minions

Without stderr=PIPE, err doesn't get picked up by proc.communicate(), so
it's not logged; rather it's dumped to the console when running tests,
interfering with the normal output.

Signed-off-by: Tim Serong <tserong@suse.com>
tests: use sequence form of Popen constructor

This eliminates the need to quote the command passed to the remote host
thus making it easier to avoid shell escaping problems, and removes any
possible security risks of using shell=True.

Signed-off-by: Tim Serong <tserong@suse.com>
tests: ssh to minions as user specified in info.yaml

This removes the hard coded `ssh ubuntu@{target}`, which makes execution
on other distros problematic.

Signed-off-by: Tim Serong <tserong@suse.com>
Merge pull request #188 from SUSE/wip-fix-server-tests

tests: fix a few problems encountered running against external calamari instance