Bug 6272 - support RESTCONF PATCH for mounted NETCONF nodes

Bug 6272 - support RESTCONF PATCH for mounted NETCONF nodes

- distinguish between operation on server or on mount point
- when operates on server then use global schema context and
data broker
- when operates on mount point then use schema context and
broker from mount point
- if broker on mount point is missing then PATCH operation fails
with global error
- contains unit tests

Change-Id: I97135a467209fcb4a5782d1c2677f482b842ff8a
Signed-off-by: Ivan Hrasko <ivan.hrasko@pantheon.tech>
(cherry picked from commit c09b7ffd0f3812a237c82805ea16a67ffdeda89d)