Swarmspawner to support new-style Docker Swarm mode

[scnerd]

Added a new class called Swarmspawner by duplicating Dockerspawner and modifying it where needed. Removed deprecated fields instead of adapting them. My working sample hub config looks like the following:

c.JupyterHub.ip = '0.0.0.0'

c.JupyterHub.spawner_class = 'dockerspawner.SwarmSpawner'
c.SwarmSpawner.image = os.environ.get('DOCKER_NOTEBOOK_IMAGE')
c.SwarmSpawner.use_docker_client_env = True

network_name = os.environ['DOCKER_NETWORK_NAME']
c.SwarmSpawner.use_internal_ip = True
c.SwarmSpawner.network_name = network_name
c.SwarmSpawner.extra_host_config = {'network_mode': network_name}
c.SwarmSpawner.remove_services = True

notebook_dir = os.environ.get('DOCKER_NOTEBOOK_DIR') or '/home/jovyan/work'
c.SwarmSpawner.notebook_dir = notebook_dir
# c.SwarmSpawner.volumes = {'notebooks/jupyterhub-user-{username}': notebook_dir}
# c.SwarmSpawner.extra_create_kwargs.update({'volume_driver': 'local'})

c.SwarmSpawner.debug = True

c.Spawner.default_url = '/lab'
c.Spawner.args = ['--NotebookApp.allow_origin=*']

c.JupyterHub.hub_ip = '0.0.0.0'
# c.JupyterHub.hub_port = 8080

c.Spawner.cpu_limit = 0.25

c.Spawner.mem_limit = '256M'

Mounting volumes not yet tested. Not sure about other other features not explicitly tested in the above configuration.

To reduce the technical debt of this addition, it would probably be worthwhile extracting a superclass for both Dockerspawner and Swarmspawner (especially their shared attributes). Most of the major differences are isolated to a few functions (start, stop, poll, get_service/container, maybe one or two others).

[scnerd]

Resolves #215

[sangramga]

Wow a really needed PR. @minrk Hope it is merged soon.

[sangramga]

@scnerd Just a doubt? Will get_ip_and_port() work with network = host (As mentioned in the docstring )

def get_ip_and_port(self):
        """Queries Docker daemon for service's IP and port.  
        If you are using network_mode=host, you will need to override  
        this method as follows::  
            @gen.coroutine  
            def get_ip_and_port(self):  
                return self.host_ip, self.port  
        You will need to make sure host_ip and port  
        are correct, which depends on the route to the service  
        and the port it `opens.`  
        """

[minrk]

Thanks, this is great! I think we should make SwarmSpawner inherit from DockerSpawner so as to avoid reimplementing too many things, but that can be done in a separate PR.

[scnerd]

I think inheritance would be great, though I think there are enough little differences in the configuration and behavior of each that I'd suggest creating a shared, abstract base class, and making both DockerSpawner and SwarmSpawner inherit from it. I meant to get around to this, but wanted to get the original SwarmSpawner tested and merged first before creating a branch that modified the main DockerSpawner.