Bug 6903 - Implement Query parameters - fields

Bug 6903 - Implement Query parameters - fields

- parse and verify fields parameter from URI
- nodes written to output are limited in
ParameterAwareNormalizedNodeWriter according to
parsed fields parameter value
- ParameterAwareNormalizedNodeWriter is used as writer
even if no parameters are used
- fields parameter is used together with depth
parameter, when node is selected by fields parameter
then value of depth parameter has no effect on selected
node and its *ancestors*
- contains unit tests for fields parameter parser and
ParameterAwareNormalizedNodeWriter class
- fixed depth for map nodes in way that when map node is
target and depth is 1 no map entries children are written
to output
- added support to read data (GET) from restconf/data path

Change-Id: I3896dba6faa3fdbcb00e07fdfd7b3fdc02ba57d9
Signed-off-by: Ivan Hrasko <ivan.hrasko@pantheon.tech>
Signed-off-by: Jakub Toth <jatoth@cisco.com>