Filter non-standard nodes from NETCONF monitoring schemas

Some NETCONF servers use augmented netconf-monitoring schema and
controller fails to parse XML with strict parsing requirement.

Add a filtering function based on the namespace of the XML node,
and filter out any node not in the netconf-monitoring namespace to
prevent parsing failure.

JIRA: NETCONF-881
Change-Id: I93b7da0baf00de59613d970fd4ec89e47eb26e58
Signed-off-by: Sangwook Ha <sangwook.ha@verizon.com>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>