Previously this option had no effect on the output. This commit makes
the option effective and also refactors the tests for this method to
cover this case.

Adds an option to the `floaty list` subcommand so that, when listing
active hosts, floaty will output just the hostnames, without any
additional information or formatting. Hostnames will be separated by a
newline.

This functionality is primarily intended for consuming by tooling (such
as the tab completion scripts).

This commit adds a new tab completion script for zsh. It also
fixes the completion script for bash to work with ABS backends.

ABS now returns the ABS job_id on the first line, then every
vmpooler hostname indented. It queries them from vmpooler to
get the metadata like lifetime etc

are returned by ABS. Added test with vmpooler and nspooler result