Provide a means to reduce duplication in YAML templates.

[simonvane]

What would you like to be added:
A means of reducing duplication in our YAML templates, perhaps with similar functionality that helm offers.

Why is this needed:
We would like to avoid mistakes due to some cases of the duplication being changed but other cases being missed.
Examples:

• Many templates define identical labels (or at least they should :-)).
• Different virtual services have the same redirect rules when in maintenance mode i.e. all redirect to a common maintenance page.
• Different virtual services differ only by name and host.
• etc.