Allow Jira REST API to be located outside of the server root

If Jira is hosted outside the server root, e.g.
http://devsrv.example.com/issues/, this change ensures that
the Jira path prefix('/issues/') is also included in all REST requests.

Based on the original idea from Ic3314355ae3d68d2ee28261ec45707dc778c3a13

Change-Id: I9a0183b5854f432196c8561bb49bb79b5ec53004