Code review comment for lp:~gmb/launchpad/enable-bz-3.4-bug-415779

This branch fixes bug 415779 by making it possible for Bugzilla.getExternalBugTrackerToUse() to recognise Bugzillas which offer an API.

I've done this by creating two methods: one which can be used to check whether the remote system has an API of the right version and one which can be used to check if the remote system has the LP plugin installed. There's a bit of duplication in these methods' try:except blocks, but I decided not to try and refactor these since doing so would make things a lot less easy to understand.