Improve Host Aggregates handle method.

Added update conditions to the handle method (remove
and add hosts).
Previously, to perform an update, first all existing hosts
were removed and then all new hosts were added.
Now, the update method only
remove or add the changed hosts.
Added missing unit tests for the update.

Change-Id: I52acdd1226be504cf2d0cf029353dbf80e4aa01a
Closes-bug: #1287192
(cherry picked from commit 8f4331a6bdc12bb7632f2d170cc94176e0897fbf)
Merge "Improve Host Aggregates handle method." into milestone-proposed