Adding gcc to the list of packages in tempest dependence

Tempest is failing due the lack of gcc to compile some required
packages with pip when it creates the virtualenv tree.
An example can be found in https://goo.gl/t0Pl3a

Change-Id: Iea725a9400080740af91cae625d3188c50bdd1a5

roles/validate-tempest/templates/configure-tempest.sh.j2

@@ -39,7 +39,7 @@ public_net_id=$(neutron net-show {{ undercloud_public_net_name }} -f value -c id
 {% endif %}
 
 # Generate a tempest configuration.
-sudo yum install -y libffi-devel openssl-devel python-virtualenv
+sudo yum install -y libffi-devel openssl-devel python-virtualenv gcc
 
 {% if tempest_source == "rdo" %}
 {% if tempest_format == "packages" %}