Prepare to use ssh inside the docker container to check if tarballs
already exist, see the next patch.
Remove the '' around UserKnownHostsFile as using SSH_COMMAND in the next
patch without rsync fails otherwise with:
command-line: line 0: Bad configuration option: 'userknownhostsfile
Change-Id: I6dfe5d587d05063c6cf2c9ec683e7b7e499b79c1
