Explicit reading of list keys using subtree filtering

Explicit reading of list keys using subtree filtering

- According to RFC-6241, section 6.2.5, NETCONF server doesn't
  have to provide values of list keys, if NETCONF client doesn't
  ask for these fields; see following statement:

  o  If any sibling nodes of the selection node are instance
     identifier components for a conceptual data structure (e.g.,
     list key leaf), then they MAY also be included
     in the filter output.

JIRA: NETCONF-735
Change-Id: I9bb048010578fd89190f616664e2175c3d3326ae
Signed-off-by: Jaroslav Tóth <jtoth@frinx.io>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>