Avoid shadowing outer `client` name in salt.cloud.clouds.nova.destroy()

************* Module salt.cloud.clouds.nova
salt/cloud/clouds/nova.py:355: [W0621(redefined-outer-name), destroy] Redefining name 'client' from outer scope (line 111)
Merge pull request #11132 from hulu/clouds-nova

Avoid shadowing outer `client` name in salt.cloud.clouds.nova.destroy()