The validate_cmd parameter is only valid on newer versions of puppet.
This module is supporting all 3.x versions of puppet. This commit only
applies the validate_cmd parameter to the file resource if it is
defined and adds documentation noting that it will not work on older
versons of puppet.
This is to work around a validation issue that arises under the 3.x future
parser, as proper numeric types have been introduced (ie, not all scalar values
are strings). Users have come to expect to to be able to pass in unquoted
integer values to the order parameter which will fail to validate as a string
when the future parser is enabled.
