Make delete a little bit handier.

Enable the ability to determine remote instances created by the user
without needing to specify instance names.

Bug: 117474343
Test: acloud delete
acloud delete --all
acloud delete --instance_names ...
atest acloud_test
Change-Id: Ia76fdb134092721b7d84a2f4c731f775d48a961c
10 files changed