Run f20 jobs in experimental-tripleo

Adds the check-tripleo-overcloud-f20 job to the tripleo
exerimental jobs where we aren't already running it.

We need this job in experimental because it is what we
run as a 'check' overcloud job for most of the TripleO
projects (it is our baseline) and it will be a useful data
point for comparing exerimental results from other projects.

I hate to waste resources on two experimental overcloud jobs but
I'm also finding it hard to argue my case (about things breaking)
without this data point.

Change-Id: Ib4685c000c950b75264be4cacdb250b4ebb47f21
Merge "Run f20 jobs in experimental-tripleo"