General Webterminal Support per Environment

Description

related to gitlab-ce#28338

It would be awesome to have per environment Webterminal support with the following assumptions:

1. allow to specify a host
2. allow to specify credentials (username + private key / password)

Let users launch terminal

Proposal

Easier integration of webterminal for non kubernetes environments

Links / references

Documentation blurb

(Write the start of the documentation of this feature here, include:

1. Why should someone use it; what's the underlying problem.
2. What is the solution.
3. How does someone use this

During implementation, this can then be copied and used as a starter for the documentation.)