Add support for "global.clusterDomain"

Noticed a hardcoded cluster.local while evaluating openDesk in clusters which have a different value in use as the internal domain name.

Used global.clusterDomain since I found the same value around the Jitsi chart. It looks reasonable and probably should be kept consistent across charts. Not sure if the Best Practices chart is intended to document also those commonly used values. If so then it would be good to add it there into the examples.

Testing

Checked via a call to helm template:

helm template .  --output-dir out --set global.clusterDomain=manual.test
helm template .  --output-dir out

Prepared openDesk integration

• bmi/opendesk/deployment/opendesk!124 (merged)