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
I really don't like these exported function vars if avoidable, which I think it is here. Can you please do it like this:

```go
var initUbuntuUser = func(...) {
}

func InitUbuntuUser(...) {
    return initUbuntuUser(...)
}
```

Then in the test you can override the unexported var/function, exporting in export_test.go if necessary.