Fix ondemand-dex package var name

[i-mtz]

Current version of the role fails if you have install_ondemand_dex set to true as in

ood-ansible/tasks/install-package.yml

Line 102 in 2698b08

name: "{{ 'ondemand-dex' if ondemand_dex_package == 'latest' else ondemand-dex }}"

there is an non valid and undefined variable name ondemand-dex instead of ondemand_dex_package.
In this case the role fails:

TASK [osc.open_ondemand : Install ondemand-dex] *****************************************************************************************************************************************************************************************************************************************
fatal: [ood-ubuntu-devel.scicore.unibas.ch]: FAILED! => 
  msg: |-
    The task includes an option with an undefined variable. The error was: 'ondemand' is undefined. 'ondemand' is undefined
  
    The error appears to be in '/home/martin0009/ansible/playbooks/playbook-scicore-int/roles/osc.open_ondemand/tasks/install-package.yml': line 100, column 3, but may
    be elsewhere in the file depending on the exact syntax problem.
  
    The offending line appears to be:
  
  
    - name: Install ondemand-dex
      ^ here

Fixes #235

[johrstrom]

Thanks for the fix!