Describe the bug
Default name for policy assignment exceed '24' characters, which results in error.

To Reproduce

• Try creating any policy.
• Creating policy assignment without name flag

  az policy assignment create \
                --scope "/providers/Microsoft.Management/managementGroups/test-group" \
                --policy "/providers/Microsoft.Management/managementgroups/test-group/providers/Microsoft.Authorization/policyDefinitions/<policy-name>"
  
  

Steps to reproduce the behavior.

Expected behavior
I should be able to create assignment of policy at given management group with some system generated name.

Environment summary
Running it in bash with az cli <2.0.60> in mac mojave.