lib, agent, client: change host_id key to int

get_stats_from_storage() previously returned a dict with the host_id as
a string.  With all valid host_id values being integers, it makes the
api cleaner to return an integer.

Change-Id: I56b204080391cf09b2606316d9195bf12905034f
Bug-Url: https://bugzilla.redhat.com/1015724
Signed-off-by: Greg Padgett <gpadgett@redhat.com>