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
1 file changed