apiclient: don't log at error level when dialing

[rogpeppe]

PR #6620 changed the client logging so that it logs an
error when it can't connect to an API address.
It is common for these errors to occur when connecting
even though the actual connect succeeds, so this
PR changes the logging to debug level - it's part
of the implementation but not something that users
should always see.

This is a backport of #6830 to Juju 2.1, as the errors
are confusing people and 2.2 won't be out for a while.

[mhilton]

👍

[jameinel]

$$merge$$

[jujubot]

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