Set default proxy preferences for existing Hosts

Currently when upgrading from 3.1 to 3.2 the pm_proxy_preferences value
in vds_static remains empty for installed Hosts.
Only new Hosts will get pm_proxy_preferences = 'cluster,dc' and this
will appear in the Host Edit popup in the Source field when the user can
change the sort of those values.
Actually, installed Hosts will still use the 'cluster,dc' value under
the hoods since this is the default when the Host has no value for
pm_proxy_preferences field.

This patch updates all Hosts that have empty pm_proxy_preferences to set
the pm_proxy_preferences value to 'cluster,dc' such that editing an
installed Host will show those values in the Edit popup Source field.

Change-Id: Ibe41451cf140c17f036e5c1ac95b3402460e7725
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1048356
Signed-off-by: Eli Mesika <emesika@redhat.com>