fix: check existing admin user on container start

[alexandrovas]

On second and subsequent container starts user already exists in database. We must check for the existence of the user, otherwise the create command will exit with a non-zero code:

root@6cbfbe92ad1f:/# su -c "gvmd --role=\"Super Admin\" --create-user=\"$USERNAME\" --password=\"$PASSWORD\"" gvm
User exists already.
root@6cbfbe92ad1f:/# echo $?
1

Same check needed for delete default admin user.