webadmin: Add max length validation to PM password

Adds max length (50 chars) validation to PM password option in Host
detail popup.

Change-Id: I30c6275f70b3692dd9fc25704b302d85e7651720
Bug-Url: https://bugzilla.redhat.com/1129553
Signed-off-by: Martin Perina <mperina@redhat.com>