Update Beaker image used as rhel-7.5 default
Update Beaker image used as rhel-7.5 default

Since the image currently used is from March 2018. This suppose
to also speedup Beaker deployments.

Change-Id: Ibdce406105bdfbc504c0f1312ece3d4f06244c31

redhat-openstack/infrared,redhat-openstack/infrared,redhat-openstack/infrared