Update the load balancer to support sticky sessions

[hoangmirs]

Why

• As we almost work on web applications that use session/cookie. So it's better to enable sticky sessions for the load balancer by default in this template.
• We can use a variable to enable/disable this feature.
• Reference: https://docs.aws.amazon.com/elasticloadbalancing/latest/application/sticky-sessions.html
• Example code: https://github.com/nimblehq/varun-api-infrastructure/blob/363310addfbda2155eafcbaa9aa02572fca00271/modules/ecs-service/target-group.tf#L9
• Note: For API applications, we don't need it as they are stateless, and the application will get the current user by the access token.

Who Benefits?

Developers that use this template

[hoangmirs]

Fixed in #120