allow aurora executor to be customized via the commandline

Right now the AuroraExecutor takes runner_provider, sandbox_provider and status_providers. These need to be the following:

• runner_provider: TaskRunnerProvider (assigned_task -> TaskRunner)
• status_providers: list(StatusCheckerProvider) (assigned_task -> StatusChecker)
• sandbox_provider: SandboxProvider (assigned_task -> SandboxInterface)

These are generic enough that we should allow these to be specified on the command line as entry points, for example, something like:

  --runner_provider apache.aurora.executor.thermos_runner:ThermosTaskRunnerProvider
  --status_provider apache.aurora.executor.common.health_checker:HealthCheckerProvider
  --status_provider myorg.zookeeper:ZkAnnouncerProvider
  --sandbox_provider myorg.docker:DockerSandboxProvider

Then have these loaded up using pkg_resources.EntryPoint. These plugins can either be linked into the .pex or injected onto the PYTHONPATH of the executor.