Modify ldap schema for hosts.

Previously all the DNS names for a given IP had an arbitrary  dn constructed
out of the first DNS name used for that IP.  That was producing various
bugs when we tried to manipulate other names other than the lucky first
name.

Now there are two different dn types for hosts.  For private/internal
hosts, the dn is of the form dc=<instanceid>.<domain>,ou=hosts, etc.

For public/floating IP hosts, the dn is of the form dc=<ip>,ou-hosts,
etc.

This patch simplifies host accessor and searcher methods accordingly.
Domain objects are now managed internally by host objects, and Domain
args have been removed from the host object interface.

Change-Id: Ia604df3317e7ba92240f02e8ae3dd0776b17ca4b