Fix duplicate resource error in host definition

If cobbler is enabled on AIO, a duplicate resource error
occurs due to a host defintion being added both by the
coe::base class and the coi::profiles::cobbler_server class.
This patche modifes the cobbler_server class to use
ensure_resource instead of creating the resource directly.

Closes-Bug: #1291068
(cherry picked from commit 97acc80b488597ab858e6cd0b83a20989e45e722)