iscsi: Use assert on checks for readability

assert will also fail when we're not meeting the conditions, makes
clear what we're actually testing, and isn't listed as a skipped task
when the condition is ok.

Change I940b1d3eceb98e16fa366c243672f588b1412d70 dropped CentOS 7,
so now we're just asserting that it's not RedHat.

Change-Id: Iec4e9a6922b67f7c9eb79f580fffbcc8160529e9

ansible/roles/iscsi/tasks/precheck.yml

@@ -26,9 +26,9 @@
     - iscsi_services.iscsid.enabled | bool
 
 - name: Check supported platforms for tgtd
-  fail:
-    msg:
-      The SCSI target daemon tgtd is not supported on CentOS/RHEL 8 and later
+  assert:
+    that: ansible_facts.os_family != 'RedHat'
+    fail_msg: >
+      The SCSI target daemon tgtd is not supported on CentOS/RHEL
   when:
-    - ansible_facts.os_family == 'RedHat'
     - enable_tgtd | bool