support configuration

[jnan806]

Signed-off-by: Jiangnan Jia jnan0806@gmail.com

Describe what this PR does / why we need it

Add configuration support

Does this pull request fix one issue?

Fixed #18

Describe how you did it

1. add option style API in NewControlPlane for other control-plane to integrate
2. Add Priority of loading config: NewControlPlane Options > SystemEnv > YamlFile > Default
   by this Priority order makes the unique value, when config-items were provided by several way,
3. If start OpenSergo directly, can also provide config-items by command args, which will adapt to NewControlPlane Options.

Describe how to verify it

1. exec ./main -h to get command args helps
2. add different values in command args, SystemEnv, yaml file

# command args : -p means give a port -c means the path of config
./main -p 8080 -c /opensergo.yaml

# SystemEnv: /etc/profile
OPENSERGO_ENDPOINT_PORT=8081

# /opensergo.yaml    filePath which was given by command above
endpointPort: 8082

Special notes for reviews

[sczyh30]

cc @123liuziming @panxiaojun233