Install lxc on Dwellers

Summary:
Salt Cloud lxc provider also need lxc-ls,
provided on CentOS by the lxc-extra package.

Test Plan:
After state has been applied state, package lxc-extra
is well installed and lxc-ls command available.

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D982