Fixes to @RadishTheHut's memory facts as discussed in https://github.com/ansible/ansible/pull/10129#issuecomment-72077500
Fixes to @RadishTheHut's memory facts as discussed in
https://github.com/ansible/ansible/pull/10129#issuecomment-72077500

* Switch default value from 0 to None.
* Prefill keys with default value so that determining calculated values
  is easier

thaim/ansible,thaim/ansible