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.

Imported from xen-api.git f3c578f0c8ce724891925c2bd9beb3be66b83e4f

Signed-off-by: Jon Ludlam <jonathan.ludlam@citrix.com>
Imported-by: John Else <john.else@citrix.com>

Conflicts:
	ocaml/xapi/api_server.ml
Merge pull request #2008 from johnelse/ca-133718-tampa-lcm

CA-133718: Fix HIMN login_with_password