Add vulnerable image search functionality

[ashmastaflash]

(this will require updates to donbot and halocelery)

Set this repo to build a container image under docker.io/halotools: https://github.com/cloudpassage-community/vulnerable_image_check

Add functionality to support the following command pattern to lexicals:

vulnerable image search [registry $REGISTRY_NAME] [image $IMAGE_NAME][:$IMAGE_TAG]

The desired outcome is that the user can ask donbot for vulnerabilities associated with images.

• If the user issues the command donbot vulnerable image search: all vulnerable images in the Halo account will be returned.
• For command donbot vulnerable image search registry MY_REGISTRY_NAME: all vulnerable images in the named repository will be returned.
• For command donbot vulnerable image search image MY_IMAGE_NAME: all vulnerable images in the halo account matching MY_IMAGE_NAME will be reported on.
• The user may combine registry and image names to refine searches, and may optionally include the tag for the image, expressed in the IMAGE_NAME:TAG format.