Fix tsh login issue

[r0mant]

If --proxy differs from the actual proxy public addr during tsh login, it fails with the following error:

$ tsh --proxy=localhost --user=alice --insecure login
WARNING: You are using insecure connection to SSH proxy https://localhost:3080
Enter password for Teleport user alice:
WARNING: You are using insecure connection to SSH proxy https://proxy.example.com:3080
error: not logged in

With the following stack trace:

ERROR REPORT:
Original Error: *trace.NotFoundError not logged in
Stack Trace:
	/home/go/src/github.com/gravitational/teleport/lib/client/api.go:578 github.com/gravitational/teleport/lib/client.StatusCurrent
	/home/go/src/github.com/gravitational/teleport/tool/tsh/db.go:138 main.fetchDatabaseCreds
	/home/go/src/github.com/gravitational/teleport/tool/tsh/tsh.go:703 main.onLogin
	/home/go/src/github.com/gravitational/teleport/tool/tsh/tsh.go:440 main.Run
	/home/go/src/github.com/gravitational/teleport/tool/tsh/tsh.go:221 main.main
	/usr/local/go/src/runtime/proc.go:204 runtime.main
	/usr/local/go/src/runtime/asm_amd64.s:1374 runtime.goexit
User Message: not logged in

This PR fixes the issue by making sure that fetching database creds uses proxy address that's been updated after login procedure, not the one originally specified on the CLI.

[russjones]

Fix looks fine, but can you add test coverage before merging.

[r0mant]

@russjones Sounds good, I've added a test for the changes in fetchDatabaseCreds. It also looks like tsh login is not being tested at all right now. Ideally we'd have tests for it as well, though it being a CLI driven flow might make it a bit challenging to test and require some refactoring. I'll make a ticket so we address it in one of future PRs.