* Set default Debian snapshots URL to CENGN
* Upgrade existing stx.conf file to match
TESTS
=======================
Run "stx config --upgrade" with various combinations of the old, new &
missing values of debian_snapshot_base and debian_security_snapshot_base
Story: 2010055
Task: 46096
Signed-off-by: Davlet Panech <davlet.panech@windriver.com>
Change-Id: Id5dd6a8054a8ac5b1a89f5bdd355450d6a7e3b1a