openstack.check: Be more careful when claiming credentials are invalid

In order to check the user's credentials at the start of deployment, we
try to run `glance image-list`. I found a situation where this command
failed despite my credentials being correct. Morph outputted a
misleading error message that said 'Wrong OpenStack credentials'

The code now checks that the error returned by 'glance' does indeed
look like a credentials error. If it doesn't, the full error output is
displayed. The error I encountered now gets a message like this:

    ERROR: openstack.check failed with code 1: ERROR: Failed to connect
    to OpenStack instance at https://example.com:5000/v2.0:
    [('SSL routines', 'SSL3_GET_SERVER_CERTIFICATE', 'certificate verify
    failed')]

(If you are curious, I fixed this by running `update-ca-certificates`.)

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