Make DEA to be conscious belonging zone and advertise it

To extend the 'zone' functionality, DEA is made to
- be conscious of which zone it belongs to, and
- advertise the zone info via message bus / varz

This is a part of a set of pull requests that adds more intelligent / convenient featurenos to 'zone' functionality.
Details of the features are given in the pull request to cloud_controller_ng [1]. Here we just give a brief description.

[1] https://github.com/cloudfoundry/cloud_controller_ng/pull/269

The set of pull requests:

- introduces finer logic to distribute instances of an app
- adds "zone validation" to enhance manageability
- configures "main zone", which is the prior place where the first instance of an app will be deployed
- enables a user to specify a single zone to deploy (all instances of) an app

The commit in this pull request provides a basis of the features above.