Avoid compiling unnecessary Janus plugins & transports

[jdeanwallace]

While looking into #7, it seems like we can get away with not compiling most Janus plugins & transports and still have WebRTC video streaming work for us.

Janus still seems to work as expected if we configured the build in the following way:

./configure --prefix=/opt/janus \
  --disable-all-plugins \
  --disable-all-transports \
  --disable-all-handlers \
  --enable-websockets \
  --enable-websockets-event-handler

This means we could also get rid of all the unused config files and the nested variables too.

I assume this would also cut down the build time. I haven't timed it, but it at least felt faster to me.

Doing something like this, might make the role useless for anything other than what we need. Also it might not be worth the time to create user friendly flags in order to keep the role generic.