Add config option to force CN

Add configuration option `forceCN` to ACME provisioner. When this option
is set to `true`, provisioner should generate Subject.CommonName for
certificate if it was not present in the request. Default value of
`false` should keep the existing behavior (do not modify CSR and
certificate).

Ref: https://github.com/smallstep/certificates/issues/259

authority/provisioner/acme.go

@@ -15,6 +15,7 @@ type ACME struct {
 	Type    string  `json:"type"`
 	Name    string  `json:"name"`
 	Claims  *Claims `json:"claims,omitempty"`
+	ForceCN bool    `json:"forceCN,omitempty"`
 	claimer *Claimer
 }