weldr: accept rhsm parameter in sources

The system sources allow specification of the rhsm parameter, but it
isn't available in the sources configured over the Weldr API. This patch
implements support for it.

internal/store/json.go

@@ -63,6 +63,7 @@ type sourceV0 struct {
 	CheckSSL bool     `json:"check_ssl"`
 	System   bool     `json:"system"`
 	Distros  []string `json:"distros"`
+	RHSM     bool     `json:"rhsm"`
 }
 
 type sourcesV0 map[string]sourceV0