Add attribute for image api workers setting

- Add an attribute for ['openstack']['image']['api']['workers']
- Adjust the template for glance-api.conf to use this attributes
- Adjust the default value to be a maximum of 8 (but this can be
  overridden)
- Adjust spec tests to set the cpu value so they converge
- Add test to confirm this value can be set
- Add test to confirm the .min[8 , sets a max of 8.
- Add README.md attribute entry

Change-Id: I8bdbd8bc8fe9af7a9ca3bbc8856d5a200ce7be7b
Closes-Bug: #1274884