Stellar-core new autoconfig code can't handle example config (Version 11.2.0)

[graydon]

In #2125 a new config was added docs/stellar-core_example_validators.cfg which, if you run stellar-core with it, fails assertion isQuorumSetSane.

This is because we defined quorum sets as sane only if they're 3 nesting levels or shallower, and the new auto-config code groups nodes by homeDomain before putting them into nesting levels. This makes for a 4-level nesting overall, not 3.

A trivial fix for this is to change the test in QuorumSanityChecker::checkSanity from depth > 2 to depth > 3 but I'm not sure if that's actually the semantics we want.