Merge ~smoser/cloud-init:fix/run-centos-unbound-var into cloud-init:master

[Scott Moser]

tools/run-centos: make running with no argument show help.

If you ran tools/run-centos without an argument it would fail due
to 'set -u' like:
  ./tools/run-centos: line 266: 1: unbound variable

[Chad Smith (chad.smith)]

+1 LGTM!

review: Approve

[Server Team CI bot (server-team-bot)]

PASSED: Continuous integration, rev:33d573907d3ffc790e28102ecac15c3be6a85462
https://jenkins.ubuntu.com/server/job/cloud-init-ci/76/
Executed test runs:
    SUCCESS: Checkout
    SUCCESS: Unit & Style Tests
    SUCCESS: Ubuntu LTS: Build
    SUCCESS: Ubuntu LTS: Integration
    SUCCESS: CentOS 6 & 7: Build & Test
    IN_PROGRESS: Declarative: Post Actions

Click here to trigger a rebuild:
https://jenkins.ubuntu.com/server/job/cloud-init-ci/76/rebuild

review: Approve (continuous-integration)

diff --git a/tools/run-centos b/tools/run-centos
index b10e3bc..d44d514 100755
--- a/tools/run-centos
+++ b/tools/run-centos
@@ -262,7 +262,7 @@ main() {
     return 0
 }
 
-if [ "$1" = "prep" ]; then
+if [ "${1:-}" = "prep" ]; then
     shift
     prep "$@"
 else