Add a trivial get_base_url() implementation to LaunchpadTrackerModel
Add a trivial get_base_url() implementation to LaunchpadTrackerModel

The purpose of get_base_url() is to permit Bug objects to
find the relevant TrackerModel. But it is not possible to
do that with Launchpad.net bug trackers, as they all share
a URL prefix.

onceuponatimeforever/oh-mainline,mzdaniel/oh-mainline,mzdaniel/oh-mainline,vipul-sharma20/oh-mainline,SnappleCap/oh-mainline,mzdaniel/oh-mainline,ehashman/oh-mainline,mzdaniel/oh-mainline,moijes12/oh-mainline,nirmeshk/oh-mainline,campbe13/openhatch,Changaco/oh-mainline,heeraj123/oh-mainline,willingc/oh-mainline,Changaco/oh-mainline,moijes12/oh-mainline,mzdaniel/oh-mainline,ehashman/oh-mainline,Changaco/oh-mainline,SnappleCap/oh-mainline,mzdaniel/oh-mainline,ojengwa/oh-mainline,ehashman/oh-mainline,onceuponatimeforever/oh-mainline,sudheesh001/oh-mainline,SnappleCap/oh-mainline,onceuponatimeforever/oh-mainline,onceuponatimeforever/oh-mainline,ehashman/oh-mainline,ehashman/oh-mainline,waseem18/oh-mainline,waseem18/oh-mainline,moijes12/oh-mainline,Changaco/oh-mainline,moijes12/oh-mainline,openhatch/oh-mainline,heeraj123/oh-mainline,waseem18/oh-mainline,sudheesh001/oh-mainline,ojengwa/oh-mainline,eeshangarg/oh-mainline,campbe13/openhatch,openhatch/oh-mainline,mzdaniel/oh-mainline,sudheesh001/oh-mainline,ojengwa/oh-mainline,openhatch/oh-mainline,SnappleCap/oh-mainline,heeraj123/oh-mainline,waseem18/oh-mainline,vipul-sharma20/oh-mainline,eeshangarg/oh-mainline,ojengwa/oh-mainline,ojengwa/oh-mainline,Changaco/oh-mainline,openhatch/oh-mainline,campbe13/openhatch,sudheesh001/oh-mainline,eeshangarg/oh-mainline,willingc/oh-mainline,willingc/oh-mainline,vipul-sharma20/oh-mainline,sudheesh001/oh-mainline,willingc/oh-mainline,heeraj123/oh-mainline,nirmeshk/oh-mainline,nirmeshk/oh-mainline,openhatch/oh-mainline,vipul-sharma20/oh-mainline,willingc/oh-mainline,SnappleCap/oh-mainline,onceuponatimeforever/oh-mainline,nirmeshk/oh-mainline,eeshangarg/oh-mainline,eeshangarg/oh-mainline,waseem18/oh-mainline,moijes12/oh-mainline,heeraj123/oh-mainline,campbe13/openhatch,campbe13/openhatch,vipul-sharma20/oh-mainline,nirmeshk/oh-mainline