Added swift as a trove prerequisite

If you want to use trove for database backups,
you need swift.

The install guide does not state this requirement,
which needs to go on the following page:

http://docs.openstack.org/trunk/install-guide/install/apt/content/trove-install.html

The need to add swift is documented in the
following trove doc scrub etherpad:

https://etherpad.openstack.org/p/trove-doc-items

I added the swift info.

Change-Id: I49102b6c961464f14484797f06949540fe400920
Closes-Bug: #1346438
Merge "Added swift as a trove prerequisite"