Cert: Run script additions

Default to host, don't require --host.
Most runs are host anyway.  In order to run on device
specify --device.

This will open a new menu for selecting
a device if devices don't exist in the config.  Otherwise,
it'll use the config settings.

Bug: 175149850
Test: cert/run --device
Tag: #gd-refactor
Change-Id: I033adc102833fd18e78e119ae59059bbff1048cb