travis-ci: prohibit uninitialized variables in managers/

The scripts in this directory rely on certain environment variables, so
fail if they are not set in order to improve the debugging experience.
The vmtest/ scripts already do it.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>

travis-ci/managers/ubuntu.sh

@@ -1,6 +1,5 @@
 #!/bin/bash
-set -e
-set -x
+set -eux
 
 RELEASE="bionic"