acloud setup: User must explicitly pass in --host to do host setup.

Installing packages can be a sensitive affair so let's only do it if the
user really wants it.

Bug: 124268982
Test: acloud setup
acloud setup --host
acloud setup --gcp-init

Change-Id: I69b565f0b26d7690a8eed748351480b2bf0536fa
diff --git a/setup/setup_args.py b/setup/setup_args.py
index 1ba34a0..8a56c35 100644
--- a/setup/setup_args.py
+++ b/setup/setup_args.py
@@ -38,7 +38,8 @@
         action="store_true",
         dest="host",
         required=False,
-        help="Setup host to run local instance of an Android Virtual Device.")
+        help="Setup host to run local instance of an Android Virtual Device. "
+             "Must explicitly set to kick off host setup.")
     setup_parser.add_argument(
         "--gcp-init",
         action="store_true",