json serialization optimization

We need to cache JsonFactory because it is costly to create it
everytime.

Bug-Url: https://bugzilla.redhat.com/1126833
Change-Id: Ib8928ba96e2c8a45d2951f96b3e1e52c0955eb9e
Signed-off-by: pkliczewski <piotr.kliczewski@gmail.com>