core, restapi: Replaced system stats query fields with constants

Replaced the hard coded String fields returning in GetSystemStatistics
query to constants, also changed the call to this query in the REST to use
the contstants when parsing the result

Change-Id: I1a2e3fc22c4c8228ec9362d1fb2a608c4db4746c
Signed-off-by: Tal Nisan <tnisan@redhat.com>