Update playbook to work with new Ansible versions

While running this playbook under Ansible 1.2 and 1.3 I've encountered a bunch of errors:

1. `ERROR: file not found: <base path>/ansible-jenkins/tasks/tasks/repo.yml`
2. `fatal: [host] => Conditional expression must evaluate to True or False: ${jenkins_install.changed}`
3. `fatal: [host] => error while evaluating conditional: {% if is_set("ldap,github,translation,preSCMbuildstep") and True %} True {% else %} False {% endif %}`

All of those errors are related to using syntax which is not supported anymore in newer versions.

This branch fixes those issues and updates the playbook to work with newer Ansible versions. It does the following:

1. Fixes the include paths.
2. Uses `when` instead of old `only_if`
3. Uses `when $variable defined` instead of old `is_set` syntax