core: Fix some Error constants of Host Activation

There is no reason the ACTIVATE word will be part of the constants.
As the $action and $type exist in these two messages, they can be
reused if needed for other operations on Host.

Change-Id: I65d5cd35c8ef4a1a318670a90ffdfb9bfce9ad8c
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>