Make platform-client talk to gatekeeper

Gatekeeper replaces the chunk of servers that platform-client previously had to talk to.

Also, there is no longer a need to create groups if they don't exist, etc.  Gatekeeper automatically has the current user as the "root" user of a specific "group".

There is also some code cleanup to reuse similar parts of code, which significantly shrinks the number of lines of code.  Tests have been updated, I don't think I lost any functional verification in the tests that I removed, but please make sure.

I am in the process of deploying the message-api and pool-whisperer components that also use gatekeeper.  Once those are done, it should be safe to move blip and clamshell over to platform-client as well.  PRs for converting those to use this version of platform client are incoming.

@jh-bate @nicolashery