core: fixing other_dc proxy preference implementation

Fencing with proxy from other_dc is actually performed by a proxy in the
same dc

This patch actually issues two things

1) the selected proxy preferences where not passed by the HostModel so
always the "cluster,dc" default was treated
2) when searching for a proxy on other_dc, hosts from the fenced host DC
where not filtered out.

Change-Id: I0de81858dc9f9b76bd27c75cb47086c884f1d6cd
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1131411
Signed-off-by: Eli Mesika <emesika@redhat.com>