provision/docker: delete ssh-agent

Not all tests are passing yet, but I'm splitting the commit anyway.

Related to #852. Not closing it until I get tests passing and remove it
from tsuru-deb.
provision/docker: fix tests for Provisioner.ExecuteCommand(Once)?

Closes #852.
docs/releases/tsr: fix typo in the 0.6.0 notes
docs/releases: start release notes for tsr 0.7.0

Yeah, hurry up :)