Allow verbosity control in capture

Currently `capture`'s verbosity is [forced to be `Logger::DEBUG`](https://github.com/capistrano/sshkit/blob/master/lib/sshkit/backends/netssh.rb#L75).

This patch allows one to override `capture`'s verbosity for a single command like this:

```
capture '<COMMAND I WANT TO SEE OUTPUT FOR WHILE IN INFO LEVEL>', verbosity: Logger::INFO
```