MIN_TOKEN_LIFE_SECONDS can't be changed

[wangxiyuan]

The MIN_TOKEN_LIFE_SECONDS is used to check that if the token will expire soon. It's used for reauthenticating. But it's hard coded and the value is 120 seconds forever. The long time request will get 401 error if it runs longer than 120 seconds. For example: A token will expire after 130 seconds and keystoneclient think that it doesn't to reauthenticate, but if the request cost more than 130 seconds, it will raise 401 error.

So it's better to make MIN_TOKEN_LIFE_SECONDS variable. Then other projects can set a sutibale value to satisfy themselves.