Hello Maintainers,

this PR adds the option to use multiple st2 packs-images.

This is a breaking change, as value st2.packs.image is replaced by st2.packs.images. Datatype changes from dict/hash to list of dicts/hashes.

This is useful, when a lot of different packs are being used, as a single container can result in multiple GB very quickly, mostly due to virtualenvs.

When using multiple st2 pack-images, the User can reduce possible downtimes due to reduced image size of new/changed containers. Also the build process can be a lot faster, if the User is providing custom packs.

Currently there is a fail message implemented in templates/deployments.yaml to prevent Users from accidential removal of their custom st2packs-image. It triggers, when st2.packs.image is defined and not empty. This should notify all Users about this change.

In #118 & #160 a centralization of packs-specific definitions for this chart was shown. As these PRs are still being discussed for other reasons, I've decided to include these centralizations in this PR.
This should make these PRs a little smaller and more compatible to each other, once they will or will not be finalized.