adding support for groups for live user

extra/templates/pre-build.d/root/VARS.txt.j2

@@ -4,6 +4,7 @@ export PNAME='{{ bdisk['name'] }}'
 export DISTPUB='{{ bdisk['dev'] }}'
 export DISTDESC='{{ bdisk['desc'] }}'
 export REGUSR='{{ user['username']|lower }}'
+export REGUSR_GRPS='{{ user['groups'] }}'
 export USERCOMMENT='{{ user['name'] }}'
 export REGUSR_PASS='{{ user['password'] }}'
 export ROOT_PASS='{{ bdisk['root_password'] }}'