[terraform] move image config to aws_ecs and add custom image variable

Summary:
This allows one to specify a custom keyserver image, making it more convenient for my case of testing keyserver images. Also moved where we specify the image to locals in aws_ecs as the image isn't keyserver primary specific and is shared also with keyserver secondary

Depends on D12862

Test Plan: terraform apply with custom image and without custom image

Reviewers: varun, bartek

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12863