[RFE] Allow to perform fence operations from...

[RFE] Allow to perform fence operations from a host in another DC

Adding to the pm_proxy_preferences field which is defaulted now to
"cluster,DC" another option named otherdc.
(The pm_proxy_preferences value is available via the UI Host New/Edit PM
TAB in the field named "source" , in the API it is under <pm_proxies>)

The default will stay "cluster,DC" and the admin can change this value
per host using the API

Feature wiki page was updated with this information:
http://www.ovirt.org/Features/Design/DetailedHostPMProxyPreferences#oVirt_3.5

Change-Id: Ifab8ec441751a33272fd7355029295bd1141c1ac
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1054778
Signed-off-by: Eli Mesika <emesika@redhat.com>