environs/cloudinit: use actual tools version in bootstrap agent.conf (1.20 backport)

The upgradedToVersion field in the bootstrap machine agent's config will now match the tools version that's it's going to use, instead of the version of the client. This avoids unnecessarily triggering upgrades at bootstrap time (fixing bug #1358078).

This is the 1.20 backport of https://github.com/juju/juju/pull/535.