Allow to configure Jira api URL

Merged username-removed-114630 requested to merge cernvcs/gitlab-ee:feature/jira_api_path into master

Cf. support request 2829

Provide a property api_url for JiraService in order to customize the API path. This is necessary if for instance Jira is not installed at the root of the web server. For user convenience, the API URL is automatically built from the Project URL using the same logic as before, so that it shouldn't be necessary for users to actually enter the API URL in most cases.

Remove the api_version property which seems redundant with the API URL. A DB migration removes api_version and initializes the API URL from project_url and api_version using the same logic as before for a seamless transition.