[Binary size] Make slimmed down Teleport binaries available

What

Per this thread -- https://goteleport.slack.com/archives/CEZH6UL64/p1635093861038200

Would be neat to have a slimmed-down binary designed for resource-constrained IoT devices that only run the SSH service.

How

Remove extra files, web assets, etc that wouldn't be required to run Teleport. Can even put the other services behind build tags to avoid compiling in a bunch of unused code.

Why

For small storage IoT devices

Workaround

Manual build possibly with our OSS. Not fully supported by Teleport potentially.