Add ceph-iscsi support
Add ceph-iscsi support

This patch adds support for installing, configuring all required
ceph-iscsi related packages and daemons. ceph-iscsi requires kernel
version 4.16 or greater.

To enable set
ENABLE_CEPH_ISCSI=True

Adds new config options
CEPH_ISCSI_TARGET_IQN - the rbd-target-gw iqn to use to export volumes
CEPH_ISCSI_API_HOST - the rbd-target-api ip to use to listen on
CEPH_ISCSI_API_PORT - the rbd-target-api port
CEPH_ISCSI_API_USER - the rbd-target-api user for auth against the api
CEPH_ISCSI_API_PASSWORD - the rbd-target-api password for auth against the api

The following packages are required and their deps
tcmu-runner - this requires kernel version 4.16 and > to work.  Ubuntu
doesn't have a prebuilt package for this, so we pull it down and compile it.
rtslib-fb - has a pypi package
configshell-fb - has a pypi package
targetcli-fb - doesn't have a pypi package.
ceph-iscsi - doesn't have a pypi package.

The reference documentaion to setup ceph-iscsi targets and daemons:
http://docs.ceph.com/docs/master/rbd/iscsi-target-cli-manual-install

Change-Id: Idfac4883b9d31e80124b8f2f1cc60dd9ce18c594

openstack/devstack-plugin-ceph