Allow for server-side commands

Ambari currently handles client-/agent-side commands; however there is no ability to handle server-side commands. Server-side commands should be specified as a task in a stage and managed along with the stage.

Use Case: Generate principals and keytabs on the Ambari server before sending the keytabs to their relevant hosts.

Implementation: To add the concept of a server-side task:

• update org.apache.ambari.server.serveraction.ServerAction to be an abstract class
  • server-side tasks must implement this class
• reuse existing host_role_command and execution_command data
  • server-side tasks are to have a role of AMBARI_SERVER_ACTION
  • server-side execution command data should be encapsulated as JSON and specify the ServerAction implementation class and any needed payload data
• org.apache.ambari.server.actionmanager.ActionScheduler and org.apache.ambari.server.serveraction.ServerActionManagerImpl need to be updated to handle the execution of server-side tasks
  • each server-side task should be executed in its own thread.
    • server_side tasks should be executed in (staged) order, serially - not in parallel
    • server_side tasks should ensure not to mess up stage ordering