[Container-tools] Created cdk label in ADB repository

[Lalatendu Mohanty]

As we have the CDK Vagrantfiles and kickstart file in ADB repository now 
[1] [2] , we are getting issue reported which are applicable to only CDK.

So to differentiate the issues which are only applicable for CDK  we 
have created a "cdk" label [3] which can tagged to those issues.

Let us know if you think it is not a good idea.

[1] 
https://github.com/projectatomic/adb-atomic-developer-bundle/tree/master/components/rhel
[2] 
https://github.com/projectatomic/adb-atomic-developer-bundle/blob/master/build_tools/kickstarts/rhel-7-cdk-vagrant.ks
[3] https://github.com/projectatomic/adb-atomic-developer-bundle/labels

Thanks,
Lala