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