[Cloud Scanner] Set image name in global variables

Users should be able to set image name here. In case docker hub access is not available, users can push the cloud scanner image to private ECR repository.
```
provider "aws" {
region = "; eg. us-east-1"
}

module "cloud-scanner_example_single-account-ecs" {
source = "deepfence/cloud-scanner/aws//examples/single-account-ecs"
version = "0.1.0"
mgmt-console-url = " eg. XXX.XXX.XX.XXX"
mgmt-console-port = "443"
deepfence-key = " eg. XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX"
}
```
It is required in CloudFormation also