serverconfig: store platform in server config

[briancain]

Prior to this commit, waypoint had no way internally to figure out what
kind of platform a server was, which meant it had to rely on the user to
pass the proper flag when doing any server operations on an existing
context. It also meant it was difficult to realize what a server
platform was when looking at existing context through waypoint context list. This commit updates that to store the context passed in from
install.

For any previous context prior to this change, we will show n/a in the
waypoint context list output since we did not store that context platform.

brian@localghost:go % waypoint context list                                                                                                                                                                                           ±[main]
    |        NAME        | PLATFORM | SERVER ADDRESS
----+--------------------+----------+-----------------
    | install-1611251878 | n/a      | localhost:9701
    | install-1611251992 | docker   | localhost:9701
  * | install-1611252817 | docker   | localhost:9701

Fixes #893

[mitchellh]

This PR is good as-is. Is the plan to use this for upgrade and uninstall then to not require the -platform flag?

[briancain]

@mitchellh - Yes! Eventually I think we can make it so that the platform flag won't be required for those commands, but we'll need users to use it at least for a while since old contexts won't have platform stored.

[briancain]

Also I was thinking the upgrade/uninstall enhancement to use this can come after the initial PRs go through since it's separate.

[mitchellh]

Also I was thinking the upgrade/uninstall enhancement to use this can come after the initial PRs go through since it's separate.

Yes definitely

[krantzinator]

🚀