Add etcd.conf configuration file

Copied verbatim from etcd's example on their Github repo:
https://github.com/coreos/etcd/blob/v0.4.6/Documentation/configuration.md#configuration-file

I'll be making some minor modifications to this to make it more generic
for the purposes of creating an apt package.
Set data directory in etcd.conf to /var/lib/etcd

Set a sensible default for the data directory in our supplied
`etcd.conf`. This is also what the `KyleAnderson/etcd` Puppet module
also uses by default:

https://github.com/solarkennedy/puppet-etcd/blob/8ac742b8502b11e5158ebc2be9b9611278d662df/manifests/params.pp#L26
Set empty discovery URL

Setting a discovery URL would prevent the first member of the cluster
from starting up as it's unable to connect to the discovery URL.

We don't need to set this as we can define etcd peers using the
`-peer-addr` option.

For more information on the discovery URL setting, see:
https://coreos.com/docs/cluster-management/setup/cluster-discovery/
Turn etcd.conf into a patch
Install etcd.conf to /etc/etcd
Add upstart script for etcd

Partly based on our statsd upstart script:
https://github.com/alphagov/packager/blob/20c93dd51ea289c02343a8b09e84f2c9c190e4b3/pkg/statsd/debian/statsd.upstart
Change etcd HTTP timeouts to floats

To prevent etcd from erroring with:

    Type mismatch for 'config.Config.http_read_timeout': Expected float but found 'int64'.
Add package maintainer scripts for etcd

Based on the maintainer scripts we have for statsd:
https://github.com/alphagov/packager/blob/master/pkg/statsd/debian/postinst

...and following the Debian Policy Manual:
https://www.debian.org/doc/debian-policy/ch-maintainerscripts.html

`apt-get purge` will cause the `/var/lib/etcd` and `/etc/etcd`
directories to be removed as you would expect. The `etcd` is never
removed on either a `remove` or a `purge` as this would cause UIDs to be
incremented unnecessarily if the package is reinstalled later.

There's some contention over the issue of whether to keep user accounts;
see this link if you're interested in the arguments for/against:
https://wiki.debian.org/AccountHandlingInMaintainerScripts

Tested in Vagrant:

    vagrant@packager:~/packager$ sudo dpkg -i etcd_0.4.6-ppa3_amd64.deb
    Selecting previously unselected package etcd.
    (Reading database ... 71086 files and directories currently installed.)
    Preparing to unpack etcd_0.4.6-ppa3_amd64.deb ...
    Unpacking etcd (0.4.6-ppa3) ...
    Setting up etcd (0.4.6-ppa3) ...
    etcd start/running, process 30773
    Processing triggers for ureadahead (0.100.0-16) ...
    vagrant@packager:~/packager$ sudo tail -f /var/log/upstart/etcd.log
    [etcd] Oct 24 14:50:43.644 INFO      | default-name: state changed from 'follower' to 'candidate'.
    [etcd] Oct 24 14:50:43.644 INFO      | default-name: state changed from 'candidate' to 'leader'.
    [etcd] Oct 24 14:50:43.644 INFO      | default-name: leader changed from '' to 'default-name'.
    [etcd] Oct 24 14:51:17.254 INFO      | default-name is starting a new cluster
    [etcd] Oct 24 14:51:17.256 INFO      | etcd server [name default-name, listen on 127.0.0.1:4001, advertised url http://127. 0.0.1:4001]
    [etcd] Oct 24 14:51:17.256 INFO      | peer server [name default-name, listen on 127.0.0.1:7001, advertised url http://127.0.0.1:7001]
    [etcd] Oct 24 14:51:17.256 INFO      | default-name starting in peer mode
    [etcd] Oct 24 14:51:17.256 INFO      | default-name: state changed from 'initialized' to 'follower'.
    [etcd] Oct 24 14:51:17.256 INFO      | default-name: state changed from 'follower' to 'leader'.
    [etcd] Oct 24 14:51:17.256 INFO      | default-name: leader changed from '' to 'default-name'.
    ^Cvagrant@packager:~/packager$ sudo service etcd stop
    etcd stop/waiting
    vagrant@packager:~/packager$ sudo service etcd start
    etcd start/running, process 30804
    vagrant@packager:~/packager$ sudo apt-get purge etcd
    Reading package lists... Done
    Building dependency tree
    Reading state information... Done
    The following packages will be REMOVED:
      etcd*
    0 upgraded, 0 newly installed, 1 to remove and 51 not upgraded.
    After this operation, 11.6 MB disk space will be freed.
    Do you want to continue? [Y/n] y
    (Reading database ... 71097 files and directories currently installed.)
    Removing etcd (0.4.6-ppa3) ...
    etcd stop/waiting
    Purging configuration files for etcd (0.4.6-ppa3) ...
    vagrant@packager:~/packager$
Release etcd (0.4.6-ppa3)
Merge pull request #78 from alphagov/73607174-etcd_upstart

[#73607174] Add upstart script and default etcd.conf to etcd package