devstack doesn't have necessary codecs installed for HLS playback

[gsong]

@muhammad-ammar reported that HLS playback is needed for JavaScript testing. However, out of the box, devstack is not able to run the demo at http://dailymotion.github.io/hls.js/demo successfully. This is because the H.264 codec isn't installed by default.

The workaround is to install the package by hand within the VM:

sudo apt-get update && sudo DEBIAN_FRONTEND=noninteractive apt-get install -y  ubuntu-restricted-extras

Long term we should convert this into a role and install into devstack.

/cc @edx/testeng @nedbat