tool: Use space after comma when formatting lists

When formating list of modules in error message, we used to do:

    foo,bar,baz

Now lists are formatted in more readable way:

    foo, bar, baz

Change-Id: I88d742b72ba57ad1de481a589ca53e71c8202176
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31784
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>