Give stack user passwordless sudo:

This is very helpful when performing automation.

Additionally, removed addition of stack user to admin group,
since that group does not exist on all systems, and
passwordless sudo eliminates the need it was addressing.

Change-Id: I727bd51acb920c6c2e66247069998b6ee4267895

elements/stackuser/install.d/50-user

@@ -4,9 +4,21 @@
 set -e
 set -o xtrace
 
-useradd -G admin -m stack -s /bin/bash
+useradd -m stack -s /bin/bash
+
 passwd stack <<EOF
 stack
 stack
 EOF
 
+
+sudo_file=/etc/sudoers.d/stack
+cat > /etc/sudoers.d/stack <<eof
+# the stack user sometimes requires passwordless
+# sudo for scripted operations.
+stack ALL=(ALL) NOPASSWD:ALL
+eof
+
+chmod 0440 $sudo_file
+
+visudo -c