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

[Cryptophobia]

From @arschles on March 15, 2016 23:35

Currently, although certs:attach <cert> <domain> will properly error out if <cert> is bogus/non-existent, it doesn't if <domain> 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

Copied from original issue: deis/workflow-cli#24

[Cryptophobia]

From @Joshua-Anderson on July 18, 2016 18:56

Does this require any client side work at all? This looks entirely server side to me.