Reuse code of old LVM share driver.
LVM Driver is 1st party Manila driver with NFS and CIFS support
and no share server support.
LVM Driver doesn't rely on Neutron, Nova, or Cinder. As such, it
is ideal for testing purposes and, after a period of maturation,
it should be useful in production environments.
Move generic driver's helpers to manila/share/drivers/helpers.py
and reuse them in LVM driver.
Implement 'ro' access, user access, extend_share function for
LVM driver.
Implements bp lvm-driver
Change-Id: Ia46c51ed400dbb0f1d87a758fb165ca711ed3d7c