We don't always need the role data, if the presumption is that we'll
be doing a pg_restore in conjunction with an ansible role which creates
all required roles. But, having a copy of the role data will never hurt!
It also gives us a straightforward way of restoring a database to a
standalone postgres instance without having to have provisioned a
dedicated VM with the relevant ansible roles.

At present we use myriad one-off per host scripts to do a pg_dump,
and they all do (or probably should do) the same thing. In combination
with setting options in the host's backup config file, I think
this single script covers all our routine pg backups.