Allow staging memory limit to be overridden

You can now supply `properties.dea_next.staging_memory_limit_mb` in
a manifest such as `cf-infrastructure-warden.yml` to override the amount of
memory for staging. This value is read in `cloud_controller_api.yml.erb:140`.

Signed-off-by: Michael Fraenkel <fraenkel@us.ibm.com>