Fix sudoers in bootstrap-servers when user != group

In some cases we may want a configuration in which the kolla user's
primary group name is not the same as their username. Doing this
currently breaks the sudoers configuration, since user entries should
reference a user, or a group prefixed with a '%'.

There does not seem to be a good reason to give root privileges to the
entire group (which sometimes may be a shared group), so let's revert to
giving only the user root privileges.

See kayobe CI test [1] in which a different user and group were
configured, leading to permission denied when using kolla ansible.

[1] http://logs.openstack.org/53/581053/2/check/kayobe-overcloud-centos/a70168e

TrivialFix

Change-Id: I677778ebd0de58df0adfa2a8705f161ec5552283