Add optional arguments to sensu_check provider. Set sane defaults.

First run of a new sensu check will not set all arguments due to them being missing on the 'create' method. The next run they will be added as optional parameters.

Changed this to check for 'undef' in the create method. If found, it will set them on the first run. 

Changed the sensu_check manifest to have Aggregate / Standalone default to undef.