Update the gate hooks to install proper docker package

The way gate_hook.sh tries to find the proper package to install is
sub-optimal, and misses in case there is another package containing the
package name as a substring.
Updating the file to work the same way as in [1].

[1] c27bb1f190

Change-Id: I726f993ef9d593f13abef55fca6cc2c0a2d353c7

dragonflow/tests/contrib/gate_hook.sh

@@ -9,12 +9,8 @@ function configure_docker_test_env {
     local docker_pkg
 
     sudo bash -c 'echo "tempest ALL=(ALL) NOPASSWD: ALL" >> /etc/sudoers'
-    if apt-cache search docker-engine | grep docker-engine; then
-        docker_pkg=docker-engine
-    else
-        docker_pkg=docker.io
-    fi
-    sudo apt-get install -y $docker_pkg
+    sudo apt-get update
+    sudo apt-get install -y docker-engine || sudo apt-get install -y docker.io
     sudo service docker restart
 }