NETCONF-514: Use exists instead of read for PUT

NETCONF-514: Use exists instead of read for PUT

The PUT operation needs to check if the path already exists
in order to return the proper response code according to the RFC
however calling exists instead of read is more efficient on the
back end.

Change-Id: Ibd7e99324c6a151aae91083e0c90bdf877161dee
Signed-off-by: Tom Pantelis <tompantelis@gmail.com>