Correct validation of {proxy,fastcgi}_cache_levels

Allow any combination of 1 and 2 in these formats: X, X:X,
and X:X:X, e.g.: "2", "2:2", "1:1:2". The old validation failed if
it wasn't an integer which didn't allow these formats.
Merge pull request #382 from 3flex/294-fix-proxy_cache_levels-validation

Correct validation of {proxy,fastcgi}_cache_levels