Feature/validate role options (#101)

Be more rigorous about validating user flags.

Only accept CREATE ROLE flags that doesn't have any params (i.e.
not ADMIN or CONNECTION LIMIT). Check that both flag and NOflag
are not used at the same time.

pkg/util/constants/roles.go

@@ -12,4 +12,6 @@ const (
 	RoleFlagNoLogin        = "NOLOGIN"
 	RoleFlagCreateRole     = "CREATEROLE"
 	RoleFlagCreateDB       = "CREATEDB"
+	RoleFlagReplication    = "REPLICATION"
+	RoleFlagByPassRLS      = "BYPASSRLS"
 )