add image_whitelist and default options form
add image_whitelist and default options form

if c.DockerSpawner.image_whitelist is specified, a default options form is constructed
with a select dropdown for choosing the image to launch.

image_whitelist is a dict of key: image, where the key is the value to be used in the form,
while the values are the actual docker images.

image_whitelist may be specified as a list, in which case it is cast to a dict of the form {image:image},
where the keys are the same as the values (users see actual image names, rather than abbreviations/descriptions).

minrk/dockerspawner,jupyter/dockerspawner,jupyter/dockerspawner,minrk/dockerspawner