(maint) tar_host defaults to yum_host

Previously, the REASSIGNMENT for tar_host was inverted, so if tar_host were
absent, it would not be reassigned. This commit fixes that error and adds a
test case to catch a regression in the behavior.
Merge pull request #275 from haus/maint/master/fixup_tar_host_yum_host

(maint) tar_host defaults to yum_host