Switch to Fedora-x86_64-20-20140618-sda

The heat-slow job using the Fedora-20 image as L2 guest, the
currently used version is affected by the heartbleed issue and
the Mb-Bios bug issue observed with emulation.

This version of the  Fedora cloud image uses the no_timer_check
kernel parameter, which solves the MP-Bios Bug issue.

The Image tested with ~3000 heat-slow job, without any issue.

Change-Id: I9ce9b7769c2d91e630b0362f2c2c6ac9066defbc
Closes-Bug: #1312199
Closes-Bug: #1297560
Merge "Switch to Fedora-x86_64-20-20140618-sda"
Merge branch 'master' of github.com:openstack-dev/devstack