logging: reducing verbosity of logging

We let common engine code to log exception stack traces. Jsonrpc code
will log stack traces only when debug level enabled.


Change-Id: Ia5bae1f36b035cd790afb75cf95f19795b648524
Signed-off-by: pkliczewski <piotr.kliczewski@gmail.com>
Bug-Url: https://bugzilla.redhat.com/1154398