cmd: export the function ReadToken

It may be useful for other uses of the cmd package that bypass
cmd.Client (`tsuru-admin ssh` is an example of that).

Related to #853.
testing: add capability to define the target on SetTargetFile
cmd/tsuru-base: fix call to testing.SetTargetFile
cmd: fix call to testing.SetTargetFile
provision/docker: fix call to testing.SetTargetFile
provision/docker: add the "ssh" command

Related to #853.
provision/docker: register the ssh command

Related to #853.
Revert "provision/docker: use os/exec to start ssh shell"

I just needed some time to think. Related to #853.

This reverts commit 1d1ab07f3697b4c71ec490ffbb0676ab7ae38a4d.
provision/docker: do not disable ECHO on SSH connections