Redis cache service

As with #2412563: amazon s3 storage service, it'd be nice to be able to provision Redis databases on a per-site basis, and inject the necessary settings into site settings.php. The redis module provides the site level functionality we'd need, and suggests the following entries in settings.php as a starting-point:

  $conf['redis_client_interface'] = 'PhpRedis'; // Can be "Predis".
  $conf['redis_client_host']      = '1.2.3.4';  // Your Redis instance hostname.
  $conf['lock_inc']               = 'sites/all/modules/redis/redis.lock.inc';
  $conf['path_inc']               = 'sites/all/modules/redis/redis.path.inc';
  $conf['cache_backends'][]       = 'sites/all/modules/redis/redis.autoload.inc';
  $conf['cache_default_class']    = 'Redis_Cache';

Again, as with S3 bucket provisioning, I'll write this in contrib. But I'll report progress in this issue, so as to provide more visibility. The initial plan is to focus on the integration points (see below), then later expand S3 into an AWS suite, and look at provisioning ElasticCache instances, etc.

I think all that'll be required initially is:

1. site-level field for redis host
2. site-level field for redis port (optional)
3. save these to the site context
4. inject them into settings.php

Additional stuff might include:

• setting a prefix per site, so that a single Redis db can be shared across an Aegir instance.

As always, feedback is welcome.