truthy: Validate options passed to 'allowed-values'

Make sure values passed in allowed values are correct ones. This is
possible thanks to previous commit, and should prevent users from
writing incorrect configurations.

yamllint/rules/truthy.py

@@ -98,10 +98,6 @@ import yaml
 
 from yamllint.linter import LintProblem
 
-ID = 'truthy'
-TYPE = 'token'
-CONF = {'allowed-values': list}
-DEFAULT = {'allowed-values': ['true', 'false']}
 
 TRUTHY = ['YES', 'Yes', 'yes',
           'NO', 'No', 'no',
@@ -111,6 +107,12 @@ TRUTHY = ['YES', 'Yes', 'yes',
           'OFF', 'Off', 'off']
 
 
+ID = 'truthy'
+TYPE = 'token'
+CONF = {'allowed-values': list(TRUTHY)}
+DEFAULT = {'allowed-values': ['true', 'false']}
+
+
 def check(conf, token, prev, next, nextnext, context):
     if prev and isinstance(prev, yaml.tokens.TagToken):
         return