jira: Configure credentials on per-server basis

From now on the global config format is

  jira:
    credentials:
      - base_prefix: string
        username:    string
        password:    string

When SalsaFlow needs to access JIRA, it looks for the best matching
credentials based on the base_prefix. The best matching credentials are
these where the longest part of the base URL matches.

Story-Id: SF-86
Change-Id: 271adf2a13