Support leaf-nodes-only in RFC8040 northbound
We have somehow lost (or never had) parity with bierman02 extension
for this option. The code to support it is in place, but due to the
legacy structure it fell off the truck in terms of parsing URIs, so
it could never be activated for a request.
Add the proper structure to support it again and define a capability
URI to let users know it is available.
JIRA: NETCONF-824
Change-Id: Ib05cc361d6bc3cc534f598276ecc84c9353f4cc2
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>