Fixes lp:1339976, lp:1270466 - manual provision, specific key + cloudint use ssh client

### Fix [lp:1270466](https://bugs.launchpad.net/juju-core/+bug/1270466)
Add the following cmd to add-machine:

```shell
juju add-machine ssh:user@10.10.0.3 -i ~/myid.pub
```

where -i is a specific identity key used to ssh into the machine. The key is also stored in the machine for future logins.

#### Tests

The custom key is passed to config.ReadAuthorizedKeys within ProvisionMachine. AuthKeysSuite already tests ReadAuthorizedKeys for passing in and reading specific keys and provisionerSuite already covers ProvisionMachine. As such, no extra tests have been added.

Tests for parsing the new parameters on the command have been added. 

### Fix [lp:1339976](https://bugs.launchpad.net/juju-core/+bug/1339976)
This PR also fixes a bug where cloudinit was not using the ssh client: cloudinit/sshinit/configure.go:63
s/known hosts/authorized keys/
%q instead of '%s'

Also, this message reads a bit awkwardly, and assumes the error is due to inexistence. How about:
    logger.Warningf("cannot add corresponding public key for %q: %v", args.SSHKeyPath, err)