Remove usage of wheels during jenkins worker provisioning.
Jenkins_worker role: clarify what the shallow-clone is and remove its dir when done.
Merge pull request #1397 from edx/benp/remove-wheels-jenkins-worker

Remove usage of wheels during jenkins worker provisioning.
use COMMON_HTPASSWD_USER and COMMON_HTPASSWD_USER

use COMMON_HTPASSWD_USER and COMMON_HTPASSWD_USER
based on:
XQUEUE_BASIC_AUTH_USER: "{{ COMMON_HTPASSWD_USER }}"
XQUEUE_BASIC_AUTH_PASSWORD: "{{ COMMON_HTPASSWD_PASS }}"
defined on roles/xqueue/defaults/main.yml