Fix ownership of virtualenv in bootstrap-servers

In I86bf5e1df3d6568c4f1ca6f4757f08a3dd22754d, creation of the kolla user
was moved to after package installation to ensure the sudo package is
installed when required. This change does not work when python
dependencies are installed in a virtual environment however - when the
virtualenv variable is set.

This change moves the ownership change of the virtualenv to after the
kolla user has been created. It also uses the kolla_user and kolla_group
variables to set the user and group appropriately.

Change-Id: I320e5d611099ad162945a98d5505a79606da0eba
TrivialFix