foreman integration - showing foreman hosts in new host dialog

This patch adds external host provider interface, with an implementation
for Foreman, to allow viewing foreman hosts from the "add new host"
dialog.

Limitations (will be addressed later):
1. Will only work with non-SSL foreman servers

Change-Id: I30ea180e477a8f0714c4dc97ec55f29be515723a
Signed-off-by: Oved Ourfali <oourfali@redhat.com>