cloudconfig: pass --connect-timeout to curl

[axw]

Description of change

Pass --connect-timeout to curl, so we don't
wait too long for a given URL when downloading
agent binaries from the controller.

QA steps

1. juju bootstrap azure
2. juju add-machine
3. observe that the machine agent comes up quicker than without the change
4. extract the curl commands from /var/log/cloud-init-output.log, run again through "time" to check that the private network URL times out in ~20 seconds.

Documentation changes

None.

Bug reference

Fixes https://bugs.launchpad.net/juju/+bug/1622531

[axw]

$$merge$$

[jujubot]

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