Previous tool behavior for retrieving and showing the host list was
linked to test case execution but there are times where we may want to
display a list of hosts for troubleshooting.  I've added a switch to the
tool to display the list and bail out, I've also refactored a bit to
make the impl cleaner.

BUG=

Review-Url: https://codereview.chromium.org/2734543002
Cr-Commit-Position: refs/heads/master@{#454904}