Description of problem:
Use ansible to install Metrics, task failed at [openshift_metrics : Start Hawkular Cassandra], and from the error info we could see some yaml parsing/ruamel related info.

TASK [openshift_metrics : Start Hawkular Cassandra] ****************************
task path: /root/openshift-ansible/roles/openshift_metrics/tasks/start_metrics.yaml:12
Using module file /root/openshift-ansible/roles/lib_openshift/library/oc_scale.py
<MASTER> ESTABLISH SSH CONNECTION FOR USER: root
<MASTER> SSH: EXEC ssh -C -o ControlMaster=auto -o ControlPersist=60s -o StrictHostKeyChecking=no -o 'IdentityFile="/root/.ssh/libra.pem"' -o KbdInteractiveAuthentication=no -o PreferredAuthentications=gssapi-with-mic,gssapi-keyex,hostbased,publickey -o PasswordAuthentication=no -o User=root -o ConnectTimeout=10 -o ControlPath=/root/.ansible/cp/ansible-ssh-%h-%p-%r MASTER '/bin/sh -c '"'"'/usr/bin/python && sleep 0'"'"''
An exception occurred during task execution. The full traceback is:
Traceback (most recent call last):
  File "/tmp/ansible_P4XL2n/ansible_module_oc_scale.py", line 42, in <module>
    import ruamel.yaml as yaml
ImportError: No module named ruamel.yaml

failed: [MASTER] (item=replicationcontroller/hawkular-cassandra-1) => {
    "failed": true, 
    "invocation": {
        "module_name": "oc_scale"
    }, 
    "module_stderr": "Traceback (most recent call last):\n  File \"/tmp/ansible_P4XL2n/ansible_module_oc_scale.py\", line 42, in <module>\n    import ruamel.yaml as yaml\nImportError: No module named ruamel.yaml\n", 
    "module_stdout": "", 
    "object": "replicationcontroller/hawkular-cassandra-1"
}

MSG:

MODULE FAILURE


This is a regression issue, and we saw a similar issue on Logging ansible install.

Version-Release number of selected component (if applicable):
OCP 3.5
Ansible master branch

How reproducible:
always

Steps to Reproduce:
1.prepare inventory file.

[oo_first_master]
MASTER ansible_user=root ansible_ssh_user=root ansible_ssh_private_key_file="/root/.ssh/libra.pem" openshift_public_hostname=MASTER

[oo_first_master:vars]
deployment_type=openshift-enterprise
openshift_release=v3.5.0
openshift_metrics_install_metrics=true

openshift_metrics_hawkular_hostname=hawkular-metrics.SUBDOMAIN
openshift_metrics_project=openshift-infra

openshift_metrics_image_prefix=registry.ops.openshift.com/openshift3/
openshift_metrics_image_version=3.5.0
~                                               

2.install necessary packages on control node.

git clone https://github.com/openshift/openshift-ansible

yum -y install -y ansible pyOpenSSL python-cryptography python-lxml
yum -y install httpd-tools
yum -y install openjdk-8-jre java-1.8.0-openjdk java-1.8.0-openjdk-devel 

3.ansible-playbook  -vvv  -i  ~/inventory  playbooks/common/openshift-cluster/openshift_metrics.yml

Actual results:
Install failed.

Expected results:
Install succeed.

Additional info:
Full log attached.
A similar issue on Logging install is observed.