Support for using Cluster Labels in AddonTemplate

[suresh-j2]

As part of AddonTemplate, it would be great to have read the values from Cluster Labels.

For example, if I set some hostname in managed cluster label, as part of addon manager, I can read that label key and use it while applying manifest.

This way, we can use the variable name and may be use cluster specific value also.

[qiujian16]

/assign @zhujian7