Update help text for block_migrate_cinder_iscsi option

As shown in change I7d7a708b231070468616ae852d81d2f8b01ba568
this test will pass if you're using new enough libvirt. According
to the change that closed the related bug:

I8fcc3ef3cb5d9fd3a95067929c496fdb5976fd41

Libvirt needs to be >= 1.2.17. We're using libvirt 2.5.0 on
xenial nodes now so that's why we can enable this test again
for master (pike).

Change-Id: I2f84b2996e8f062930add348668f50c9c66dda9d
Related-Bug: #1398999
diff --git a/tempest/config.py b/tempest/config.py
index b8a91b7..e23cc99 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -392,9 +392,9 @@
                      "migration"),
     cfg.BoolOpt('block_migrate_cinder_iscsi',
                 default=False,
-                help="Does the test environment block migration support "
-                "cinder iSCSI volumes. Note, libvirt doesn't support this, "
-                "see https://bugs.launchpad.net/nova/+bug/1398999"),
+                help="Does the test environment support block migration with "
+                "Cinder iSCSI volumes. Note: libvirt >= 1.2.17 is required "
+                "to support this if using the libvirt compute driver."),
     cfg.BoolOpt('vnc_console',
                 default=False,
                 help='Enable VNC console. This configuration value should '