Revert task_state on failed instance actions

Right now, the task_state logic in compute/api.py can be broken, such
that instances can get stuck in an uneditable state if an action is
performed that fails. The task_state remains something like 'stopping'
even though the action has not been completed or queued, and further
requests that depend on task_state will fail (see check_instance_state()
in compute/apy.py).

The only way out of it is to delete the instance or hack the database.

This patch adds a reverts_task_state() decorator to compute/manager.py,
which, upon operation failure, reverts the instance's task_state back
to None.

It also adds a test_state_revert() test to verify that all the actions
marked for state reversion do the right thing. It also corrects several
other tests that specifically expect the task_state to remain after
an error has occurred.

Fixes bug 1025722

Change-Id: Id4358c508156c713cb953dfa0f01a6f598bc1e7d

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