Add string interpolation linting

[csmarchbanks]

Add some way to detect when people try to use string interpolation such as '%s' but do not add % <vars> after the string.

Possible ideas:

1. Parse generated yaml for places where %(.*)s type strings are left in.
2. Improve and use the jsonnet linter

See comments in https://github.com/kubernetes-monitoring/kubernetes-mixin/pull/189

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