Sync arc::client between bluegrass and babylon.

Add a call to arc::voms to arc::client, as used on babylon. This is
missing on bluegrass but all nodes in bluegrass that use arc::client
also use arc::voms, so this is no change in practice.

Also add a couple of comments.

Not actually a change here, but a note: the list of packages from
bluegrass is the one here in the code. The list of packages installed
on babylon is lacking nordugrid-arc-plugins-globus and
nordugrid-arc-plugins-gfal but it should be OK to begin adding those on
babylon machines as well.