[Reviewer: Andy] Fix Plivo install process and configuration

Andy, please can you review my fix to the Plivo install process.  It consists of 3 changes.

* Install libevent-dev - one of the updated Python eggs now depends on it, but the Plivo install script doesn't install it.

* Install Plivo twice - there seems to be a bug in the version of sysconfig.py we have that it replaces any /usr/local with /usr - I have no idea why it does this, but it doesn't seem to do it in the latest code.  My workaround is to install in /usr/plivo (which succeeds) and then reinstall in /usr/local/plivo (which finds some files from /usr/plivo, and so succeeds, and actually installs in the right place).  All configuration files are read from /usr/local/plivo, so there's no issue with duplicating files.  The additional disk space is pretty minimal.

* Tweak some configuration to fix media (following Mike's testing).

I completely agree that installing Plivo twice is hideous, but I'd prefer not to pretty this up unless we really have to - each test cycle takes a couple of hours (because Freeswitch takes so long to install).