Support sensitive in execute resources.

Currently, passwords can be leaked to the terminal in execute resources:
```
  * execute[add to htpasswd] action run
    - execute htpasswd -b /etc/httpd/riak_htpasswd riak *REDACTED*
```
With this change, when an execute resource is marked as *sensitive*:
* streaming output will be suppressed
* the command line will not be displayed

The output now becomes:
```
  * execute[add to htpasswd] action run
    - execute sensitive resource
```

closes #1744