Adding aws vars for common role to work

The AWS vars need to be added because the common/templates/etc/logrotate.d/hourly/edx_logrotate_tracking.j2 calls the variable {{ aws_s3_sync_script }}
The symlink is being created in the aws role itself when it's run.  
```
- name: create symlink for s3 log sync script
  file: >
    state=link
    src={{ aws_s3_sync_script }}
    dest={{ COMMON_BIN_DIR }}/{{ aws_s3_sync_script|basename }}
  when: AWS_S3_LOGS
```