CA-133718: Fix HIMN login_with_password

HIMN had been relying on login_with_password working on the slave
as it was erroneously not being forwarded. When xapi was changed
to disallow logins on slaves, this broke. This changeset causes
login_with_password to be forwarded to the master, which should
fix the issue.

Signed-off-by: Jon Ludlam <jonathan.ludlam@citrix.com>
Merge pull request #1717 from jonludlam/CA-133718

CA-133718: Fix HIMN login_with_password