Expose parameter names as String constants

Expose parameter names as String constants

Remove static methods in favor of proper constants. This allows us to
use switch statements instead of if/else. Since we are at it,
differentiate between unknown and known-but-invalid parameters.

JIRA: NETCONF-773
Change-Id: Id2a9b09f8cbb5688d1722fc7ee932be4c46ceac6
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>