internal/server: refactor install into one package

[krantzinator]

Foundation to addressing #672 and #612
There's a lot of diff from moving existing code around, so I'll highlight actual changes here:

• Moved the platforms to implement an Installer interface, replacing the case setup in InstallCommand to set the platform
• Took serverinstall.Config out of InstallCommand and gave each Platform its own config
  • This was the main place I went in circles with; I wanted each Platform to have its own config, and only care about the flags it needed to. I tried out a separate serverinstall/config package; toyed with using serverconfig; moved the InstallFlags pieces from InstallCommand to serverinstall to separate packages and back again. I also worked on this piece with a couple teammates, experimenting with different ways to have generic flags and install them to the scoped platforms as part of init. Ultimately I decided to go with the Platform-specific flags you see here in order to overcome the various hurdles discovered.

Small pieces:

• fixed a bug in the k8s statefulset; we weren't actually using the value set in the pull-policy flag, and updated the flag to have an updated value of IfNotPresent to match k8s default (ImagePullPolicy: apiv1.PullPolicy(c.ImagePullPolicy),
• threw a strings.ToLower on the platform name for UX

Copying Evan's example of including examples in lieu of a CLI testing framework:

$ waypoint-dev install -platform=nomad -nomad-dc=dc1 -accept-tos
✓ Nomad allocation ready
✓ Configuring server...
Waypoint server successfully installed and configured!
...

$ waypoint-dev install -platform=docker -accept-tos
✓ Installing Waypoint server to docker
✓ Server container started!
✓ Configuring server...
Waypoint server successfully installed and configured!
...

$ waypoint-dev install -platform=kubernetes -accept-tos
✓ Creating Kubernetes resources...
✓ Kubernetes StatefulSet reporting ready
✓ Waiting for Kubernetes service to become ready..
✓ Configuring server...
Waypoint server successfully installed and configured!
...

[mitchellh]

Yeahhhhhhh this is way cleaner.

Just a couple tiny comments but this is good work.

[briancain]

📦 🎉