Add test to validate the custom grains in pillar/id/*.sls
    
    The test validates that the data that are in pillar/id/*.sls match the
    acceptable values as defined in pillar/valid_custom_grains.yaml. The
    data are separated in two groups, global and localized. The global ones
    are checked only if they are defined and if they have valid values. The
    localized ones are also checked based on the defined "country" grain of
    the minion that have realistic values (eg the city matches the country)