Fix problem with 'pools' command

There was a problem with a mismatch between the command and the
dCache message.  The command operates on a single poolgroup but
the dCache message accepts multiple poolgroups.

The solution is to create a Set object containing the pool requested
by the user.  Set is a subclass of Iterable, so satifying the
message's constructor.
Remove billing service from system test as it is currently broken and
prevents other services from starting.
Merge branch 'fix/jana/test1/omit_billing' of https://github.com/paulmillar/dcache into tutorial/test1