Add botframework to supported_hubot_adapters in ansible vars/main.yaml

[yadhu621]

When you supply 'botframework' as a value for st2chatops_hubot_adapter, the playbook fails with the following message. "st2chatops_hubot_adapter" must be of the supported adapters in https://github.com/StackStorm/st2chatops/blob/master/st2chatops.env