Add NestingDepth linter

Change-Id: I36633ca978206b61bf84b4cbe15f9f51bbcbf1c3
Reviewed-on: http://gerrit.causes.com/43623
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>
Clean up NestingDepth linter

Change it to be enabled by default, up the default max_depth to 3,
simplify the logic of the class itself, and add more tests.

Change-Id: I501a28d69556b64d22cebc828cd31a6adcddbe11
Reviewed-on: http://gerrit.causes.com/43624
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>