XD-1811 Custom module registry location

- In addition to `xd.module.home` and `classpath:/modules` as the default
module registry locations, an additional `xd.customModule.home` is provided
to let the user define any specific supported URL path for the module registry.
- Add an entry for `xd.customModule.home` in servers.yml so that user can
specify it.

Fix description not to use http:// location

Handle customModuleHome placeholder

 - Add `isActive()` to ModuleRegistry
 - Include only the active module registries to the DelegatingModuleRegistry
Comment out properties
Assign null default for custom module registry

 - Having `null` as the default value for custom resource module
registry will let us handle the look up methods return null/empty
values.
Remove redis/kryo stuff that is not used anymore
XD-2170: NoNodeException for job creation

When an admin process is elected leader/supervisor, it will query
the state of all streams and jobs. It does this by iterating all
streams under /xd/deployments/streams/[stream-name] and jobs under
/xd/deployments/jobs/[job-name]. Under each path, it is expecting a
modules node which contains all of the modules deployed for the stream or job.

The issue is that if *no* modules were deployed for the stream/job,
that node will not exist because it is created lazily by the container.
This is unlikely in the case of streams since a stream contains at
least two modules, but is much more likely with jobs since they
only contain one module.

The fix is to ensure that the
/xd/deployments/streams/[stream-name]/modules path (and the same for
jobs) is created upfront by the supervisor prior to creating the
module deployment requests. This will allow the supervisor to see
that no modules for the stream/job have been deployed and thus
take appropriate action.
XD-2119,XD-2143,XD-2144

* Add 'spring-boot-starter-security' as a dependency
* Set up the admininstration and management endpoints as insecure by default
* Unit tests for SSL
* Unit tests for security enablement

Disable AuditAutoConfiguration by default
XD-2147 Update Spring Boot to 1.1.7.RELEASE
XD-2145 - Shell changes:

* Use Apache HttpClient for authentication in the Shell
* admin config server' now supports the '--username' and '--password' parameters
* Supporting unit tests for HTTPS and authentication in the Shell.
Adding a null sink

- Null sink can be useful when the main stream isn't focused on
  stream destination but the tap streams are used for analytics etc.,
- It is also useful to iteratively add in steps to a stream
  without worrying about having to land data anywhere.
XD-2181 Add configuration options to servers.yml

* Add configuration options to servers.yml
* Update test configurations to use the scheme suggested in servers.yml