Change hard-coded actions path

Change hard-coded actions path

There was an assumption about the basePath ('rests') when creating
path for POST method for actions. The path was incorrect if instead of
the default basePath a custom one was used.

This patch fixes the path creation while not making any assumptions
about the basePath. The path is resolved when needed.

JIRA: NETCONF-1021
Change-Id: I4b4a7f8f69626874d9c9821205d7c726dfe90919
Signed-off-by: Šimon Ukuš <simon.ukus@pantheon.tech>
(cherry picked from commit 206fb9f0c4b589222fc4a9fc3a47c9e203110d70)