Be able to override the default secure vars file.

Also always pass in the common.yml vars to plays.
Add common as a dependency of splunk and datadog roles.
Use a var instead of using the number directly.
Accept a mongo uri instead of a host,port and password.
Add an explaination for the count logic.
Merge pull request #642 from edx/feanil/abbey_common

Feanil/abbey common