Demo: Rename keys

[awwad]

The keys used in the demo are in demo/keys. The director repositories' targets role key is named just "director". This is potentially confusing and should be updated.

[vladimir-v-diaz]

I found this confusing as well. It wasn't explained/clear in the code why 'director' was used as the name, but I concluded it must have been for a reason because it seemed deliberate.
https://github.com/uptane/uptane/blob/a3136cab96b77dbf77155b5c1c965ed74fecce58/demo/demo_director.py#L92

[awwad]

It's historical. Very early on in a proof of concept demo, I think I used just one key for the Director repository.

I'm making this issue more general, given our recent conversation. The image repo keys in the demo are named 'maintargets', 'mainroot', etc. This is also historical and probably misleading.

Keys should probably be updated to something like:

• director -> director_repo_targets
• directorroot -> director_repo_root
• directorsnapshot -> director_repo_snapshot
• directortimestamp -> director_repo_timestamp
• mainroot -> image_repo_root
• maintargets -> image_repo_targets
• mainsnapshot -> image_repo_snapshot
• maintimestamp -> image_repo_timestamp

These are a bit long. If you have other suggestions, that'd be good. (: