Action params now validated at the worker/uniter level

Before the Action is run as a hook on the unit, its passed params are
validated against actions.yaml.  If the validation fails, no error is
produced but the hook will log an error and fail.
Addressed concerns for PR 311
Merge pull request #311 from binary132/actions-runhook-validate

Action params now validated at the worker/uniter level

Before the Action is run as a hook on the unit, its passed params are
validated against actions.yaml.  If the validation fails, no error is
produced but the hook will log an error and fail.