Fix 'ewc_license' conditional check in stackstorm.yml example play

[arm4b]

Closes #237
Under some circumstances ewc_license could be None in stackstorm.yml Ansible play

Fixes:

fatal: [ubuntu16]: FAILED! => {"msg": "The conditional check 'ewc_license is defined and ewc_license | length > 0' failed. The error was: Unexpected templating type error occurred on ({% if ewc_license is defined and ewc_license | length > 0 %} True {% else %} False {% endif %}): object of type 'NoneType' has no len()\n\n"}

[lingfish]

Heh, I just did a PR for this as well. I believe that, if you guys have chosen to always set a default in vars, when the defined check is redundant.