Revert "Set default-ansible-version to 2.7 for all tenant (minus zuul)"

This reverts commit dff7fccc13.

We believe we've fixed the roles to work with ansible 2.8 and wish to
undo this pin now.

Change-Id: Ib6ffae651891bc85aa3a946f60085c5f5805c977

zuul/main.yaml

@@ -1,6 +1,5 @@
 - tenant:
     name: opendev
-    default-ansible-version: 2.7
     max-nodes-per-job: 10
     report-build-page: true
     source:
@@ -73,7 +72,6 @@
 
 - tenant:
     name: kata-containers
-    default-ansible-version: 2.7
     max-nodes-per-job: 1
     report-build-page: true
     source:
@@ -111,7 +109,6 @@
 
 - tenant:
     name: openstack
-    default-ansible-version: 2.7
     max-nodes-per-job: 10
     report-build-page: true
     source: