cmd/modelcmd: remove SetAPIOpener

[rogpeppe]

We have both SetAPIOpen and SetAPIOpener which are mutually redundant
and confusing. SetAPIOpener is barely used, so remove it and mock
api.Open instead, which means that more logic is tested.

Also move the NewTimeoutOpener logic into the kill command, the
only place that uses it, so we avoid using the APIOpener abstraction
in that case. At some point in the future, it would be nice to make
juju.NewAPIConnection cancellable (for example by passing
in a context.Context) which would make the connection timeout
logic more generally available.

Also add a modelcmd.InnerCommand function so that we can
inspect fields in wrapped commands without undue shenanigans.

QA no regressions; in particular, check that we can still use the juju kill
command to destroy a non-responsive controller.

[cmars]

Please do not land this unless you're sure it won't break romulus commands.

[mhilton]

LGTM

[rogpeppe]

!!build!!

[jameinel]

I can't speak to Casey's concern about Romulus, both otherwise LGTM

[rogpeppe]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju