Make cli arg parsing compliant with requirements.

Remove duplicate definition of GRADLE_OPT_TEST and rename arguments as
discussed in
https://github.com/elastic/x-plugins/pull/4137#discussion_r89660620

Default to `check` if no argument is passed, but exit with error in an
unsupported parameter is provided.

Also ensure the pipefail option is only set when the executing shell
is *really* bash as opposed to POSIX sh which doesn't support it.

Original commit: elastic/x-pack-elasticsearch@64bd740295

dev-tools/ci

@@ -13,24 +13,24 @@
 
 # Turn on semi-strict mode
 set -e
-set -o pipefail
-
-# Set default test scenario, equivalent to passing the cli arg `tests`
-GRADLE_OPT_STRING="--info check -Dtests.network=true -Dtests.badapples=true"
+if [ -n "$BASH" ]; then
+    set -o pipefail
+fi
 
 # Allow the user choose different test through a single cli arg
-if [ $# -ge 1 ]
-then
-key="$1"
+# default to `check` if no argument has been supplied
+key=${1-check}
 case $key in
-    packagingtests)
+    packagingTest)
         GRADLE_OPT_STRING="--info -Pvagrant.boxes=all :x-plugins:qa:vagrant:packagingTest"
         break;;
-    tests)
+    check)
         GRADLE_OPT_STRING="--info check -Dtests.network=true -Dtests.badapples=true"
         break;;
+    *)
+        echo "Unsupported cli argument $1. Allowed parameters are packagingTest or check. No argument defaults to check."
+        exit 1;;
 esac
-fi
 
 SCRIPT="$0"