Enable live block migration when using iSCSI volumes

Fix for bug 1160323.

DocImpact
Depends on a version of XAPI supporting relax-xsm-sr-check.
Currently no release versions support this, so the change at
https://github.com/xen-org/xen-api/pull/1116 would need to be applied
to the source to enable this.  XenServer 6.2 is expected to
support this mode, and possibly some future hotfixes for XenServer 6.1.

It also depends on a configuration change which must be documented and added to devstack:
* Set "relax-xsm-sr-check = true" in /etc/xapi.conf

This commit makes the following changes:
* Attach the SR on the destination host prior to migrate
* Returns the SR-ref from the pre_migrate call (API change)
* Populates the XS VDI map with the sr-ref on the destination host
* Removes the connection to the SR from the source host post-migrate
* Added plugin to determine if iSCSI block migration is enabled

Associated tempest test at https://review.openstack.org/#/c/26478/

Change-Id: I917d9cf094190d636f4b9e14f6c8e728ff85af0e
Fixes: bug 1160323
Fix zookeeper import and tests

Import zookeeper.membership rather than zookeeper.membersip.
Also fixed some issues with setting up the tests for the zookeeper
servicegroup driver.  Config options were not being set before
initializing the driver leading to failures.

There is no added test for this because the bug is indistinguishable
from not having the zookeeper python modules installed, which leads to
skipping these servicegroup tests.

Bug 1177776

Change-Id: Idd6dca2e03169399b930cc1fc1a85636497cb0b5
Merge "Fix zookeeper import and tests"
Merge "Enable live block migration when using iSCSI volumes"