Support master_key_required parameter at skygear.op

[rickmak]

Current way of doing it is

@skygear.op('user:forgot-password:test', key_required=True)
def func():
    access_key_type = current_context().get('access_key_type')
    if not access_key_type or access_key_type != 'master':
        raise SkygearException(
            'master key is required',
            skyerror.AccessKeyNotAccepted
        )
    # Logic

Example code if we add master_key_required

@skygear.op('user:forgot-password:test', master_key_required=True)
def func():
    # Logic

• Skygear Server Date/Version: v1.0, 10 July, 2017
• Application Platform:
• Is this a regression?
• Attached logs, screenshots