Decrease dea memory when staging/starting app

CC uses dea_advertisement to get knowledge of DEA status, which includes
the info like available memory and app instance count. By default, these
ad will be updated in 5 seconds, but when staging or starting new instance
of app, only the app instance count has increased, we need to decrease the
available memory of the DEA to let CC get more accurate info of DEAs

There are two places to decrease the available memory, one is staing, the
other one is starting application at specific index
Decrease stager memory when staging/starting app

CC uses stager_advertisement to get knowledge of stager status, includes
the info like available memory. By default, these ad will be updated in
5 seconds, but when staging or starting new instance of app, the available
memory of the stager is not decerased. We need to decrease the available
memory of the stager and let CC get more accurate info of stagers

This commit will keep consistent with the previous DEA memory decrement. In
the previous commit, it just dcrease the DEA's memory while in this one, it
decreases the stager's memory
Merge branch 'update_dea_memory' of https://github.com/xingzhou/cloud_controller_ng into xingzhou-update_dea_memory
Reserve correct memory for staging task

  * DEA reserves memory for staging task as maximum between configured
1 GB and app memory

[#64251570]
Inject dea pool in app stager task

  * App stager task needs reference to dea pool to reserve memory
in dea pool when app is staging

[#64251570]
Sends staging to Diego if config flag is set

- To use the new Diego staging pieces, just set "diego: true" in config.
- Currently, we still use NATS for sending messages to the Diego
  stagers.
- Messaging to the Diego stagers is undirected rather than directed.

[#63918294]
Enforce maximum disk quota

Signed-off-by: Ruth Byers <rbyers@pivotallabs.com>