Stringify $evaluation_interval for validate_re call

Under Puppet 3.7.x, calls to validate_re fail if the parameter is not
strictly a string.  This change creates a string from
$evaluation_interval when passing to validate_re, so the call will
pass regardless of if the parameter to the class is specified as
string or an integer.

Change-Id: I17b82977f21483acf134837a66c5ecebe346cfdf
Closes-Bug: 1371235
Merge "Stringify $evaluation_interval for validate_re call"