Revert to using curl to download tools

aria2 is in universe, which would preclude
juju from entering main. aria2 is also not
available on the test machines, and broke
CI. Revert to using curl, but cycle through
the URLs until one succeeds.

Fixes https://bugs.launchpad.net/juju-core/+bug/1364438
Merge pull request #661 from axw/lp1364438-curl-download-tools

Revert to using curl to download tools

aria2 is in universe, which would preclude
juju from entering main. aria2 is also not
available on the test machines, and broke
CI. Revert to using curl, but cycle through
the URLs until one succeeds.

Fixes https://bugs.launchpad.net/juju-core/+bug/1364438
ActionComplete / ActionFailed called after Action completion

The worker now calls ActionComplete or ActionFailed on state, depending
on the success state of the Action and any errors.

The hook is set to an error state if the Action hook is missing, or if
other errors take place during hook execution.  If the Action hook sets
a fail state, the Hook will not error, rather the Action will fail.
ActionComplete / ActionFailed called after Action completion

ActionTags are now properly initialized on worker uniter.HookContext at the
time it is created.
ActionComplete / ActionFailed called after Action completion

Final bits and pieces to complete PR 617 put in place; tests fleshed out
and passing.
ActionComplete / ActionFailed called after Action completion

Addressed davecheney's points in PR 617.
ActionComplete / ActionFailed called after Action completion

Addressed further points in PR 617:
 - Action results no longer a pointer, thus no nil checks needed
 - ActionFail call-home now sends a sane message to the user, without
   discarding error context
 - Minor tweaks to relation write-out logic
 - Removed pointless actionStatus type
 - Checks for nil state handle for ActionComplete / ActionFail
worker/uniter finalizeContext refactor for ActionFinish
ActionComplete / ActionFailed called after Action completion

Addressed remarks in PR #617:
 - Action-related HookContext types bundled into a single type.
 - Comment added to State handle in HookContext.
 - actionData constructor called from outside the HookContext, in the
   case of Actions only.
ActionFinish called after Action completion

Removed actionFailed() helper function, made naming tweak.