Support aliases for default discovery address and cluster name

Ambari service discovery should support aliases for defining the default discovery address and cluster name.

Some admins would like to be able to omit these details from descriptors when most of the descriptors will reference the same Ambari cluster.

With the remote alias service, Knox HA deployments are able to manage such aliases more consistently.

The new aliases will be named:

• ambari.discovery.address - The default Ambari discovery address

• ambari.discovery.cluster - The default Ambari cluster name

The naming convention follows the pattern of the existing ambari.discovery.user and ambari.discovery.password aliases.

If these aliases are defined, and the corresponding properties are not specified in a descriptor, the values of these aliases will be used as defaults.

If either of these aliases is NOT defined, and is also unspecified in a descriptor, then discovery shall not be attempted.

The Admin UI must also be addressed as part of this, since there is currently some validation to ensure that a sufficient amount of discovery information is specified in a new descriptor (if any is specified at all).