Fix exec_command property to be overridable as node attribute

[HironariYo]

Description

To utilize the command with post_install_action property, we should make exec_command value from kill to exec. But the value is not adjustable with override attribute. To fix this, it should have selective value with attribute on the resource.

Issues Resolved

246

Check List

• [x] All tests pass. See https://github.com/chef-cookbooks/community_cookbook_documentation/blob/master/TESTING.MD
• [ ] New functionality includes testing.
• [x] New functionality has been documented in the README if applicable
• [X] All commits have been signed for the Developer Certificate of Origin. See https://github.com/chef-cookbooks/community_cookbook_documentation/blob/master/CONTRIBUTING.MD