Refactor OpenApiTestUtils methods

Refactor OpenApiTestUtils methods

Change getPathParameters into getPathGetParameters and add
getPathPostParameters.

We assume that all paths present in OpenApi should have at least
GET example in the documentation.

Thus we can use GET operation as a base operation to get number of
parameters in path. We just need to drop 'content' parameter.

For actions and RPCs we can use getPathPostParameters method.

JIRA: NETCONF-1101
Change-Id: I2a0a24a5a567d7dd95b7adf6ec13dddb391fab51
Signed-off-by: Ivan Hrasko <ivan.hrasko@pantheon.tech>