Use nginx version from our PPA everywhere

* remove nginx Apt repo so that later versions aren't installed
* use our version (from ubuntu repo)

This is preparatory work so that we can use nginx-extra and thus use
lua/Perl to do stuff.
Merge pull request #584 from alphagov/feature/consistent-nginx-version

Use nginx version from our PPA everywhere
Be explicit about our NTP servers

*.uk.pool.ntp.org servers come from http://www.pool.ntp.org/en/use.html
and are A or AAAA records with a short TTL which point to different
servers. Also seem to use Anycast (although my networking knowledge is
sketchy).

ntp.ubuntu.com is a similarly short-lived A record.

time.euro.apple.com is what my Mac uses ;)
Merge pull request #585 from alphagov/feature/ntp-server-list

Be explicit about our NTP servers
Add a request ID to all the things

As person responsible for operating software in production
I would like to have a unique ID associated with every request
So that it is easy for me to track down problems in a distributed
system.

This is the first step to that. Every request coming in to our SSL
termination machines will have a UUID assigned to it with an HTTP
header.

We can then amend our logging to include this header where appropriate
and make it easier to trace requests going through the system.

This removes nginx-full and replaces it with nginx-extras.