OVS events: Fix configure script to detect YAJL tree API

OVS events: Fix configure script to detect YAJL tree API

If old YAJL version is installed on a system (ver < 2.0), the OVS plugin
compilation will fail as it requires tree API to be supported in YAJL
library. For this reason, it was decided to change 'configure' script
to detect YAJL tree API also.

Clean-up according updated collectd style:
  https://github.com/collectd/collectd/pull/1931
  https://github.com/collectd/collectd/issues/1951

Change-Id: I90c82cdc9780ee8c0c9b794986662a39a5ab0011
Signed-off-by: Mytnyk, VolodymyrX <volodymyrx.mytnyk@intel.com>