Stop devtest if user was not in libvirtd group

The current mechanism for picking up the new group doesn't work
correctly anyway (see bug report for a full example of the current
behavior), and it's more friendly to stop immediately and let the
user know what they need to do instead of continuing on and
failing, which buries the relevant message further up the
scrollback.

This change is largely work done by bnemec, with some additional
checking added by polleyj in order to not break a CI environment (ie, to
bypass the local group checks if there's a supplied $TE_DATAFILE which
stipulates remote operations)

Change-Id: Ia6784076700717f5c03e349be25aeac1c5c30002
Closes-Bug: 1285285
Co-Authored-By: Ben Nemec <bnemec@redhat.com>
Merge "Stop devtest if user was not in libvirtd group"