Allow connecting to self-signed quantum endpoints

Introduce a new option 'quantum_api_insecure', matching the existing
'glance_api_insecure'. It skips the SSL validation in quantumclient.

DocImpact

Change-Id: I275517179929710bedcef5086a040b9396c0e932
xenapi: Cleanup detach_volume code

Refactors `detach_volume` so that it uses the same code to detach that
detaching all volumes uses.

Change-Id: Idb7acb136367d8ad512d4b14c775c69404f6f6b0
Merge "Allow connecting to self-signed quantum endpoints"
Merge "xenapi: Cleanup detach_volume code"