This patch implements the following fixes to the bootstrap-ansible
script in order to make it work successfully on CentOS7:
- Remove yum check-update as it returns 0 only when no updates are
available.
- Add a variable to adjust the virtualenv CLI based on the distro
used as '--always-copy' does not work on CentOS.
Change-Id: Ia591c3558bd00945dc3076ed19d4c6e364bec939