Allow to evacuate without specifying a target host
When the evacuate is run without specifying a target host, horizon
sets an empty string for target host. But the evacuate api doesn't
allow an empty string. As a result, nova returns "HTTP 400 Bad
So this patch sets None as the target host when it isn't specified.
(cherry picked from commit f9e0f8a976b82088ef095a69cd1fa892cddde3ba)