Define a single, test-fakable /var/lib/juju path for all provider-related code.
Missing import.
Missing import.
Merge work into colo branch.
Remove reference to defaultRootDir.
Missing import.
Run “make simplify” to clean up the code a bit.
[r=jtv] Run "make simplify" to clean up the code a bit.

We should do this regularly. Perhaps it's time to make this part of our
standard code formatting, to take it one step up from plain gofmt.
Review change: single function in environs/testing to override DataDir in tests.
[r=jtv] Make "/var/lib/juju" path a variable.

That particular path, for cloudinit's benefit, is hard-coded all over the
provider code. It may be an obstacle to testing, because tests can't
substitute a fake (and obviously tests shouldn't mess with /var/lib/juju, or
even rely on its existence).

If we later find that one of these mentions is different from the others, it's
quite possible that we have to specialize some of the uses into separate
variables. For now, the main things are to be aware, and to have an injection
point for tests.