Clean-up regarding "2.5.1.1" and prep for 2.5.1.2

2.5.1.1 was originally an internal release which was eventually
created differently. We are now using "2.5.1.1" to refer to the
cumulative roll-up of 2.5.1 hotfixes. As such, this commit deletes the
(stale, unused) rel- and override- manifests for 2.5.1.1, and adds an
"official" released/2.5.1.1.xml manifest reflecting what was actually
shipped. Finally, this updates rel-2.5.1.xml to reference the upcoming
2.5.1.2 build.

Change-Id: I84cba4fdecb5698793bcfc0f6e7115deeed8d168
Reviewed-on: http://review.couchbase.org/42189
Reviewed-by: Dave Finlay <dave.finlay@couchbase.com>
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Tested-by: Chris Hillery <ceej@couchbase.com>