Add targeted config loading to swift-init

This allows an easier and more explicit way to tell swift-init to run on
specific servers.  For example with an SAIO, this allows you to do
something like:

swift-init object-server.1 reload

to reload just the 1st object server.  A more real world example is when
you are running separate servers for replication.  In this example you
might have an object-server/public.conf and
object-server/replication.conf.  With this change you can do something
like:

swift-init object-server.replication reload

to just reload the replication server.

DocImpact
Change-Id: I5c6046b5ee28e17dadfc5fc53d1d872d9bb8fe48
Merge "Add targeted config loading to swift-init"