Split the database grants into their own class

In anticipation of the nodepool patch, make the database grants,
which need a running database server to run, their own class. This
allows us to skip running them while in the chroot, and then on
boot we can simply run:

  puppet apply 'class {"openstack_project::slave_db": }'

Change-Id: I4223e691d7545b445d39048a8ca801458cc3c2ed
Merge "Split the database grants into their own class"