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
```

I sort of don't like the 'command' variable here when its not always the actual command, also this could be a bit more ruby:

```ruby
command_str = @new_resource.sensitive ? "sensitive resource" : @new_resource.command
```

if you don't like the ternary, its still clearer to return the value from the if statement.