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.
Btw, golang conventions is to let comments start with the name of the function/method. So here eg. 

```
// validateAction validates the given action params against the spec defined for the charm.
```