Replace "tele login" with "tsh login"

[benarent]

Objectives

• tele login command is no longer needed to login into Hub.
• tsh login is used to login into Hub and regular clusters.
• tele commands (such as build, ls, pull, push) work after tsh login.
• tsh login can login users into Docker/Helm as well.

Implementation notes

• tele should be taught to read user's x509 certificate from ~/.tsh and use it for client auth when talking to Gravity API.
• Gravity web servers (pack/app/ops) should be extended to support client-cert auth and extract user information from the client certificate. Maybe use Teleport's lib/auth/middleware.go:AuthMiddleware for that.