add chart prom rules to template

[kevin-lindsay-1]

Description

this commit exposes up the prometheus alert rules as configurable on the faas-netes/openfaas chart.

you may specify a list of rules, which by default are added as additional rules following the default rules. you may optionally set includeDefaults to false in order to override the existing defaults.

Motivation and Context

• [x] I have raised an issue to propose this change (required)

How Has This Been Tested?

helm template and looking at the output yaml files for each of the 4 cases:

• [x] include defaults, no rules
• [x] include defaults, rules
• [x] do not include defaults, rules
• [x] do not include defaults, no rules. this just outputs a []

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I've read the CONTRIBUTION guide
• [x] I have signed-off my commits with git commit -s
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[derek[bot]]

Thank you for your contribution. I've just checked and your commit doesn't appear to be signed-off. That's something we need before your Pull Request can be merged. Please see our contributing guide. Tip: if you only have one commit so far then run: git commit --amend --signoff and then git push --force.