Fix "sudo: sorry, you must have a tty to run sudo"

On many systems the requiretty sudoers option is turned on by default.
With "requiretty" option the sudo ensures the user have real tty access.

Just several "su" variant has an option for skipping the new session creation step.

Only one session can posses a tty, so after a "su -c" the sudo will not
work.

We will use sudo instead of su, when we create the stack account.

This change adds new variable the STACK_USER for
 service username.

Change-Id: I1b3fbd903686884e74a5a22d82c0c0890e1be03c
diff --git a/tools/xen/prepare_guest.sh b/tools/xen/prepare_guest.sh
index 4aa4554..fe52445 100755
--- a/tools/xen/prepare_guest.sh
+++ b/tools/xen/prepare_guest.sh
@@ -19,6 +19,7 @@
 STAGING_DIR=${STAGING_DIR:-stage}
 DO_TGZ=${DO_TGZ:-1}
 XS_TOOLS_PATH=${XS_TOOLS_PATH:-"/root/xs-tools.deb"}
+STACK_USER=${STACK_USER:-stack}
 
 # Install basics
 chroot $STAGING_DIR apt-get update
@@ -46,12 +47,12 @@
 
 # Add stack user
 chroot $STAGING_DIR groupadd libvirtd
-chroot $STAGING_DIR useradd stack -s /bin/bash -d /opt/stack -G libvirtd
-echo stack:$GUEST_PASSWORD | chroot $STAGING_DIR chpasswd
-echo "stack ALL=(ALL) NOPASSWD: ALL" >> $STAGING_DIR/etc/sudoers
+chroot $STAGING_DIR useradd $STACK_USER -s /bin/bash -d /opt/stack -G libvirtd
+echo $STACK_USER:$GUEST_PASSWORD | chroot $STAGING_DIR chpasswd
+echo "$STACK_USER ALL=(ALL) NOPASSWD: ALL" >> $STAGING_DIR/etc/sudoers
 
 # Give ownership of /opt/stack to stack user
-chroot $STAGING_DIR chown -R stack /opt/stack
+chroot $STAGING_DIR chown -R $STACK_USER /opt/stack
 
 # Make our ip address hostnames look nice at the command prompt
 echo "export PS1='${debian_chroot:+($debian_chroot)}\\u@\\H:\\w\\$ '" >> $STAGING_DIR/opt/stack/.bashrc