Create schema on update if it does not exist

Makes the `cmgr update-schema` subcommand and cmgrd
`POST /schemas/{schema_name}` function like upserts
rather than updates. If the named schema does not
exist, a warning will be logged and the schema will
be created, rather than returning an error.

This allows clients to call `update-schema` without the
need to know whether a given schema has already been
created or not, without needing to handle nonexistent
schema errors and falling back to `add-schema`.

cmgr/api.go

@@ -350,7 +350,8 @@ func (m *Manager) UpdateSchema(schema *Schema) []error {
 	if err != nil {
 		return []error{err}
 	} else if !exists {
-		return []error{fmt.Errorf("schema '%s' does not exist", schema.Name)}
+		m.log.warnf("schema '%s' does not exist, creating...", schema.Name)
+		return m.CreateSchema(schema)
 	}
 
 	return m.convergeSchema(schema)