ENH: Check for empty header variables.
ENH: Check for empty header variables.

Checks to see if _OFFLINE_POOLS_GREP_STRING and
_TEMPLATE_GUESTS_GREP_STRING are empty. If they are the UUID from
_UUID_GENERAL_USE is used in place so that the dependant calls to these
variables do not fail. grep is one such case where an empty variable
would cause a failure. A UUID is used due to the statically low
possibility of a collision with an actual pool name.

chyves/chyves