Add option to deregister node before draining.

[somnusfish]

Issue number: #156

Description of changes:

• Add DEREGISTER_FROM_LB_WAIT_TIME_IN_SEC in .env as environment variable.
• Deregister node from load balancer before draining when DEREGISTER_FROM_LB_WAIT_TIME_IN_SEC is larger than 0.
• Register node to load balancer after draining when DEREGISTER_FROM_LB_WAIT_TIME_IN_SEC is larger than 0.

Testing done: Tested on cluster, monitored node get labeled and unlabeled.

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[somnusfish]

Address the comments with name change and updated readme.md.