libvirt: cleanup network volume driver auth config
libvirt: cleanup network volume driver auth config

The LibvirtNetVolumeDriver is handling both rbd and
sheepdog (iscsi) connections. The auth config logic
is mingling both backends which is really confusing.

For example, the iscsi protocol only defines auth_method,
auth_username and auth_password. It does not set an
auth_enabled value in the connection_info['data'] dict.

This change simplifies the logic involved for setting
the auth config by decoupling the rbd/iscsi handlers.

A follow-up change will build on this to fix the
rbd auth config to prefer the cinder volume connection_info
auth data over the local config for nova in the case
that different cinder backends are used for ephemeral
and block storage.

Change-Id: I8a55d87f75ecad757ce81b1f5f77c3a551154a17
Partial-Bug: #1635008

rahulunair/nova,rajalokan/nova,Juniper/nova,jianghuaw/nova,mikalstill/nova,Juniper/nova,gooddata/openstack-nova,klmitch/nova,vmturbo/nova,klmitch/nova,hanlind/nova,mahak/nova,klmitch/nova,hanlind/nova,rajalokan/nova,rajalokan/nova,phenoxim/nova,mahak/nova,jianghuaw/nova,gooddata/openstack-nova,rahulunair/nova,mahak/nova,vmturbo/nova,vmturbo/nova,vmturbo/nova,phenoxim/nova,gooddata/openstack-nova,Juniper/nova,rahulunair/nova,Juniper/nova,openstack/nova,klmitch/nova,hanlind/nova,openstack/nova,gooddata/openstack-nova,openstack/nova,rajalokan/nova,mikalstill/nova,mikalstill/nova,jianghuaw/nova,jianghuaw/nova