[CHEF-5163] Support lazy evaluation the mount resource's options attr

set_or_return calls #call on any DelayedEvaluator on read and compares
the results against the parameter validations.  This patch adds String
to the evaluation but ensure that someone reading the options value
will only ever get an Array, keeping the original contract.
[CHEF-5163] Add tests for delayed evalutors in mount options
Merge pull request #1356 from opscode/ssd/CHEF-5163

[CHEF-5163] Support lazy evaluation the mount resource's options attr