lava: add flexible options for unpack with tar

For AUM tests unpack of overlay with tests in verbose mode
produce too lot of almost useless information. This is the reason
why the job log exceeding 5MB limit and should be downloaded as
plain text which is hard to analyze.

Added common macros `tar_unpack_cmd()` which create dynamic unpack
string depending on variable `tar_additional_options` ('-v' by default).

Additional verbose option for `tar` was switched off for all AUM tests
to simplify log analysis in WEb interface.

It is possible to override default options with `lqa` command, for
example: `lqa -t tar_additional_options:""`.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>