parameterize various master/infra scale group bits

allow selectively overriding master scale group bits:
instance type: openshift_aws_master_group_instance_type
scale group min size: openshift_aws_master_group_min_size
scale group max size: openshift_aws_master_group_max_size
scale group desired size: openshift_aws_master_group_desired_size

and overriding infra scale group bits:
instance type: openshift_aws_infra_group_instance_type
scale group min size: openshift_aws_infra_group_min_size
scale group max size: openshift_aws_infra_group_max_size
scale group desired size: openshift_aws_infra_group_desired_size

and overriding compute scale group bits:
instance type: openshift_aws_compute_group_instance_type
scale group min size: openshift_aws_compute_group_min_size
scale group max size: openshift_aws_compute_group_max_size
scale group desired size: openshift_aws_compute_group_desired_size

also consolodate the health check for master/infra/compute since they are all the same

Default to just cleaning out all the objects in the S3 bucket (IFF openshift_aws_create_s3 is 'true').

If you really, trully want to delete the S3 bucket and free up the bucket name, you can set openshift_aws_really_delete_s3_bucket to 'true' ('false' by default).

Automatic merge from submit-queue.

un-hardcode default subnet az

allow defining 'default_az' for one of the subnets in the vpc structure, and pull that default_az and set it to openshift_aws_subnet_az.
    
this should allow one less variable to be defined/overridded when using non-default (us-east-1) regions

Fix to master-internal elb scheme

allow defining 'default_az' for one of the subnets in the vpc structure, and pull that default_az and set it to openshift_aws_subnet_az.

this should allow one less variable to have to be defined/overridded (openshift_aws_subnet_az) when using non-default (us-east-1) regions

update provisioning_vars.yml.example to show an example VPC structure

add deprovisioners/uninstallers for objects created via playbooks/aws/openshift-cluster/prerequisites.yml

specifically: security groups, vpcs, and any ssh keys

introduce openshift_aws_enable_uninstall_shared_objects to protect AWS objects that would be shared in the case of multiple clusters co-existing in one AWS account. right now it protects the ssh keys, but it can/should be used to protect against deleting the shared IAM instance profile as well. default this variable to False to be on the safe side when uninstalling/deprovisioning.

add some documentation on using deprovisioning playbooks