cluster-launch-installer-e2e*: Drop AMI override

Since openshift/installer@66ef3beb (*: remove support for container
linux, 2018-09-06, openshift/installer#221), the installer defaults to
a recent RHCOS AMI.  Drop the override here, so we can track RHCOS
with only installer-repo bumps.  I've left the variable in the
template so that, if this repo needs to have its own, separate,
opinion again, it can set the override again.  However, to support the
no-local-opinion case, the new local-opinion syntax will be:

  export EC2_AMI_OVERRIDE='ec2AMIOverride: ami-07307c397daf4d02e'

ci-operator/templates/cluster-launch-installer-e2e-smoke.yaml

@@ -189,7 +189,6 @@ objects:
           export [email protected]
           export PASSWORD=$( date +%s | sha256sum | base64 | head -c 32 ; echo )
           export SSH_KEY="$( cat /etc/openshift-installer/ssh-publickey )"
-          export EC2_AMI_OVERRIDE=ami-07307c397daf4d02e
           export EXPIRATION_DATE=$( date -d '4 hours' --iso=minutes --utc )
           envsubst </etc/openshift-installer/openshift.yaml >/tmp/cluster/inputs.yaml
         )

ci-operator/templates/cluster-launch-installer-e2e.yaml

@@ -193,7 +193,6 @@ objects:
           export [email protected]
           export PASSWORD=$( date +%s | sha256sum | base64 | head -c 32 ; echo )
           export SSH_KEY="$( cat /etc/openshift-installer/ssh-publickey )"
-          export EC2_AMI_OVERRIDE=ami-07307c397daf4d02e
           export EXPIRATION_DATE=$( date -d '4 hours' --iso=minutes --utc )
           envsubst </etc/openshift-installer/openshift.yaml >/tmp/cluster/inputs.yaml
         )

cluster/test-deploy/aws/openshift.yaml

@@ -9,7 +9,7 @@ aws:
   # assetsS3BucketName:
 
   # (optional) AMI override for all nodes. Example: `ami-foobar123`.
-  ec2AMIOverride: "${EC2_AMI_OVERRIDE}"
+  ${EC2_AMI_OVERRIDE}
 
   external:
     # (optional) List of subnet IDs within an existing VPC to deploy master nodes into.