[r=axwalk],[bug=1263586] environs/cloudinit: force symlink of juju-run
Manual provisioning fails if the juju-run symlink
exists. We should always recreate the symlink if
it exists, by passing -f to ln.
Also: moved symlinking of juju-run to the machine
agent, so that upgrades do the right thing.
Fixes #1263586
https://codereview.appspot.com/47380044/