Describe what this PR does / why we need it

unify configuration

Describe how you did it

The priority is: env > yaml > default
That is say, yaml config will override default config and env config will override yaml config.

There is only one parameter "config path" for sentinel initialization. If user don't indicate config path, sentinel will use default config.

Unifying the constant to core/constant, including configuration、global constant variable.

FIX #42

Describe how to verify it

Special notes for reviews