Remove duplication of brick.iscsi in brick.iser

TargetAdmin and TgtAdm in iser is mostly a copy of TargetAdmin and
TgtAdm in iscsi.

Change-Id: I8d89a35c478485370b41f4dfd376de07b9fc9314
Nexenta drivers ignore "does not exist" exception

Ignore "does not exist" exception in NFS and iSCSI driver,
delete_volume and delete_snapshot methods.
This exceptions means that the volume or the snapshot not exists in the
backend, so we can avoid "delete" errors, when the resource was already
deleted.

Closes-Bug: 1240650
Change-Id: I3b2684097dcb95ba50a67c94a353dc3f17374777
Merge "Nexenta drivers ignore "does not exist" exception"
Merge "Remove duplication of brick.iscsi in brick.iser"