soft-qemu handling with F20 could image

When the qume used with Linux image without
a para-virtualized timer, various timing issues can happen,
if the qemu process preempted in the wrong time, for example
at timer test.

The issues less like happens on low load, high core number
host system, but it can happen.

For soft qemu (TCG) generally recommended to explicitly disable the
timer check.

Pre-caching the Fedora `gate edition` image, which contains the
the no_timers_check option.

Related-Bug: #1297560
Partial-Bug: #1312199

Change-Id: Id5cd01a92a047b7859914e5bb017c15ee443b4d5
Merge "soft-qemu handling with F20 could image"
Merge branch 'master' of https://github.com/openstack-dev/devstack