Do not land code on blocked branches

[Ian Booth]

> 
> Curtis has filed three new bugs for these so far, and there appears to
> be one or two more to come:
> 
> <https://bugs.launchpad.net/juju-core/+bug/1450912>

The issue here is that quickstart is relying on Juju 1.16 behaviour. There was a
block of code with a comment:

// Remove this whole if block when 1.16 compatibility is dropped.

The code block was removed because 1.18 was our minimum compatibility version.
But it seems we have to restore the 1.16 behaviour. Note that this is not an
upgraded environment where we need to retain compatibility with older
deployments. It is a fresh 1.24 install which should be able to rely on 1.18 and
later behaviour only.