In a single controller scenario, the "Upgrade status check result"
does nothing because the previous task can only succeed when
`nova-status upgrade check` returns code 0. This change allows this
command to fail, so that the value of returned code stored in
`nova_upgrade_check_stdout` can then be analysed.

This change also allows for warnings (rc 1) to pass.
Closes-Bug: 1834647

Change-Id: I6f5e37832f43f23604920b9d890cc505ca924ff9