If two NayuOS images are built on the same day, the second one will not get zipped. The -f flag in gzip can be used to overwrite the previous image

/reviewed-on nexedi/slapos!336

/reviewed-on nexedi/slapos!223

-  add --nouse-image flags to cros_sdk to prevent usage of missing lvm2
-  add dynamic discovery of ebuild files instead of rely on hard-coded versions

/reviewed-on nexedi/slapos!219

grep '----- something' fails because of the '-' character
used for passing options:
grep: unrecognized option '----- something'
so only grep for the string 'something'

Automate ebuild revision numbers.
Reactivate testing.

Desactivate testing temporary.

/reviewed-by @vpelletier