[sensu_check] validate subscribers/standalone on create action only

[cwjohnston]

Description

This change introduces conditional logic to validate subscriptions or standalone settings on sensu_check resources only when the action is :create.

Motivation and Context

Closes #536

How Has This Been Tested?

Added unit test coverage.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.