Fix patch target parsing

Fix patch target parsing

Our tests were not updated for RFC8040, hence they are written with
weird codecs which utterly do not match RFC8072 definition of
yang-patch.

The problem is that patch edit's target is interpreted in terms of an
extension to the URL, i.e. it does NOT follow JSON nor XML parsing
rules, nor is it anything in-between -- we must use ParserIdentifier's
facilities.

Update the parsers and the test suites, ditching a lot of complexity
in process of doing so.

JIRA: NETCONF-804
Change-Id: If9d9c77e0e2397ffdcb8e8c0f0fef0b9b02bac8b
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>