Add 'ANSIBLE_ROLES_PATH' in ansible_env

Add 'ANSIBLE_ROLES_PATH' in ansible_env

JIRA: YARDSTICK-1024

This work adds the 'ANSIBLE_ROLES_PATH' as one of the Ansible environment variables.
This 'ANSIBLE_ROLES_PATH' is set to the roles directory in Yardstick repo.
By adding this variable, Ansible will first searches the local project for a role, then searches the roles_path.
It will let the users of the plugin projects to able to use a relative role path for any of their own roles in ansible playbook.

Change-Id: I0926ab0c0fa5c6cc37c4fe560db5b2aec3fe6eb1
Signed-off-by: JingLu5 <lvjing5@huawei.com>