Allow resource limits to be specified for Warden in the director config

When using bosh-lite to run processes that need many open file descriptors (eg Cassandra, Jenkins etc.), the default 1024 limit is insufficient. This allows us to change these limits within the warden containers by adding them to the director.yml:

```yml
---
name: Bosh Lite Director
# ...
cloud:
  plugin: warden
  properties:
    warden:
      unix_domain_socket: "/tmp/warden.sock"
      rlimits:
        nofiles: 10240
# ...
```

All options that can be specified within the rlimits property are defined in [Warden::Protocol::ResourceLimits](https://github.com/cloudfoundry/warden/blob/master/warden-protocol/lib/warden/protocol/pb/resource_limits.proto)
Pretty sure these were the warden container limits. The way we used to handle this before was to set ulimits within each job monit starts, but that meant making changes in many places. The real problem was with products that we didn't own, e.g. Cloud Foundry. The above addition means that we can increase the global container ulimit which will be respected by all processes started within those containers.