Allow customizing the default label (OPS-3868)
You don't want to use this in almost any case other than when using NagiosCheckDict which
in turn reports performance values for multiple checks, and in that case you want to
differentiate between the various default values, instead of having them all be named
'value'
Merge pull request #18 from Work4Labs/olefloch/olefloch/OPS-3868/fix_perf_value_name

Allow customizing the default label (OPS-3868)