cmd/jujud: fixed unreliable TestLoginsDuringUpgrade test

This test has been unreliable since the introduction of the
certupdater worker. This worker restarts the API server after upgrades
have completed, often breaking the API connection used by this test
(timing dependent).

Instead of starting the whole machine agent, TestLoginsDuringUpgrade
now just starts the specific workers required for the test, avoiding
the unwanted effects of other workers. This also makes the test
significantly faster.

Some refactoring of the machine agent was required to support running
of the specific workers required.

This fixes LP #1400358.
Merge pull request #1301 from mjs/1400358-TestLoginsDuringUpgrade-broken

cmd/jujud: fixed unreliable TestLoginsDuringUpgrade test

This test has been unreliable since the introduction of the certupdater worker. This worker restarts the API server after upgrades have completed, often breaking the API connection used by this test (timing dependent).

Instead of starting the whole machine agent, TestLoginsDuringUpgrade now just starts the specific workers required for the test, avoiding the unwanted effects of other workers. This also makes the test significantly faster.

Some refactoring of the machine agent was required to support running of the specific workers required.

Fixes LP #1400358.

(Review request: http://reviews.vapour.ws/r/615/)
Fixed lp:1401130 - networker is disabled for joyent environments
Merge pull request #1308 from dimitern/lp-1401130-joyent-disable-networker

Fixed lp:1401130 - networker is disabled for joyent environments

Networker was causing problems on Joyent environemnts.
Since http://pad.lv/1401130 is a CI blocker, this is
a temporary fix that disables the networker for Joyent.

I've filed a separate low-priorty bug http://pad.lv/1401423
to propely fix the networker for joyent, when possible.

Live tested on Joyent.

(Review request: http://reviews.vapour.ws/r/623/)
Action supercommand

Use JUJU_DEV_FEATURE_FLAGS envar with an option for "action" to enable
this supercommand.

For example:

   export JUJU_DEV_FEATURE_FLAGS="${JUJU_DEV_FEATURE_FLAGS}, action"
make sure feature flag for action is set in TestHelpCommands
Test with and without feature flag
fixes for feedback trivials