Fix recursive delete of host

When host.delete(recursive=True) is called, it would
previously delete all services that contained host.host_name.

Now deletion is slightly smarter, and if there is a service
with service.host_name="host_a,host_b" and host_a is deleted,
then only the reference to host_a is deleted, not the whole service.
Assert that the service is on two hosts before host delete
Revert "Assert that the service is on two hosts before host delete"

This reverts commit aaad3720c04a7d58f25cbaf5a48f66a0e839e3fe.

.filter only returns the one result.
Merge pull request #162 from pynag/fix-recursive-delete-of-host

Fix recursive delete of host