Fix coverage for add_computehost with ID of host

add_computehost returns HostNotInFreepool if UUID is given for host
because freepool.hosts only contains names of hosts

Change-Id: I7ea9b240fa7beb60c532175794da2f30a551a985
Closes-Bug: #1261788
Merge "Fix coverage for add_computehost with ID of host"