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/debian.sh

@@ -21,7 +21,7 @@ function docker_exec() {
     docker exec $ENV_VARS -it $CONT_NAME "$@"
 }
 
-set -e
+set -eu
 
 source "$(dirname $0)/travis_wait.bash"
 

travis-ci/managers/ubuntu.sh

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