Update legacy variable substitution syntax deprecated in ansible 1.2

Using the-ansibles with Ansible 1.4+ results in deprecated syntax warnings – unless you've turned off the warnings! – due to old style variable substitution syntax being used. I note that several roles use the old style syntax and I'm updating them as I use them.

This pull request is only for the timezone role. Let me know if you'd like a bunch of the fixed roles in one pull request instead of a pull request per roll...