Addition of PostgreSQL::Test::Cluster::pg_version()

Hi all,
(Added Andrew in CC.)

While working more on expanding the tests of pg_upgrade for
cross-version checks, I have noticed that we don't expose a routine
able to get back _pg_version from a node, which should remain a
private field of Cluster.pm.  We already do that for install_path, as
of case added by 87076c4.

Any objections or comments about the addition of a routine to get the
PostgreSQL::Version, as of the attached?

Thanks,
--
Michael