Get commentLink w/ association from project.config

Align its-base behaviour with Gerrit hierarchical configuration of
commentLink:
- project.config with inheritance
- gerrit.config as fallback

With regards to the association policy, on project.config is taken
from the plugin-specific section.

Example of gerrit.config association policy:
[plugin "its-jira"]
   association = MANDATORY

Change-Id: I7dadb6f0bd6b1952fdc18d8c24ead36cca33ae9d