Fix undefined variable for repo installs

[sandipb]

https://github.com/ansible-community/ansible-consul/pull/460 introduced an typo causing repo installations to fail

TASK [ansible-consul : Install OS packages] ***********************************************************************************************************************
fatal: [neptune]: FAILED! => {"msg": "The conditional check '(consul_os_repo_prerequisites)' failed. The error was: error while evaluating conditional ((consul_os_repo_prerequisites)): 'consul_os_repo_prerequisites' is undefined\n\nThe error appears to be in '***/.cache/roles/ansible-consul/tasks/install_linux_repo.yml': line 4, column 3, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n\n- name: Install OS packages\n  ^ here\n"}

Also reported here: https://github.com/ansible-community/ansible-consul/issues/466