determine if domain is valid in 'certs:attach <cert> <domain>'

Currently, although `certs:attach ` will properly error out if `` is bogus/non-existent, it doesn't if `` is similarly non-existent. Consider the following unit test, which is currently failing (err is nil):

```
if err := Attach(&client, "test-example-com", "non-existent.domain.com"); err == nil {
t.Fatal("An Error should have resulted from the attempt to attach a valid cert to a non-existent domain")
}
```

This issue tracks work on the client side, and deis/workflow#475 tracks server-side work