Fix parsing of fields parameters

Fix parsing of fields parameters

- Multiple nested schema nodes splitted by semilcolon can be placed
  on one level of QNames, also, when the nested schema nodes are
  specified by multiple-nodes path; example:
  fields=service(type;instance/name;instance/provider)
- Previously, multiple-nodes nested path to schema node had to be
  listed as the last parameter in brackets without any other
  multi-nodes nested path.

Change-Id: I0d2f76047342e19a996fea0abe4001597d1c0a03
JIRA: NETCONF-320
Signed-off-by: Jaroslav Tóth <jtoth@frinx.io>