wip: proposed changes to ITE7 from contribfest

[mikhailswift]

These were my notes from my meeting with @adityasaky and @SantiagoTorres at In-toto's Contribfest session:

• Testing section needs to be filled out
• Infrastructure section perhaps should address testing infrastructure added as part of the ITE7 implementation
• Prototype implementation section needs to be updated to reflect the current implementation in in-toto-golang
• Condense rootcas & intermediatecas to a singular trust bundles object on the layout

Included in this PR is a first pass at updating the ITE 7 document to reflect the changes discussed here. Once we have a consensus on the way forward I can begin updating the in-toto-golang implementation of this ITE.

[adityasaky]

Relevant to #33

[SantiagoTorres]

Discussed during the community meeting