This change fixes the empty value set to the gid option in rsyncd.conf,
which was caused by reference to the invalid USER_GROUP variable, and
ensures the option is set to the group which STACK_USER belongs to.
This also fixes duplicate declaration of the local user_group variable.
Closes-Bug: #1940742
Change-Id: Ifd0a5ef0bc5f3647f43b169df1f7176393971853