Fix rhel optional repo not found and not reported as error

error links:
http://pastebin.test.redhat.com/872688
http://pastebin.test.redhat.com/872689

problem 1)
currently even a made up repo enable command returns zero:
[stack@undercloud-0 ~]$ sudo yum-config-manager --enable petepan-repo-rhel-7
[stack@undercloud-0 ~]$ echo $?
0
we can switch to using "shell" ansible module, and grep for the baseurl
so that the if it fails we'll actually catch the error

problem 2)
no such thing as rhel-7-server-optional-rpms repo (exists only in som latest versions)
what does exist is : rhelosp-rhel-7.8-server-opt
and we can use : rhelosp-rhel-*-server-opt
to be version agnostic

Change-Id: Icd5c62d3d421e0f561032620ab3548d32b4ff387

roles/tobiko-ensure-python3/tasks/install.yaml

@@ -2,8 +2,8 @@
 
 - name: "enable yum repos"
   become: yes
-  command:
-    yum-config-manager --enable '{{ item }}'
+  shell:
+    yum-config-manager --enable '{{ item }}' |grep baseurl
   loop: '{{ enable_yum_repos }}'
 
 

roles/tobiko-ensure-python3/vars/RedHat-7.yaml

@@ -1,7 +1,7 @@
 ---
 
 enable_yum_repos:
-  - rhel-7-server-optional-rpms
+  - rhelosp-rhel-*-server-opt
 
 python_packages:
   python3: