/reviewed-on nexedi/slapos.toolbox!63

/reviewed-on nexedi/slapos.toolbox!51

Also set database connection timeout to 5 second

Add the possibility to choose unit between byte or percent,
and to choose between checking used or free memory.

avoid to return exit code (int) or mem (object)
better return a tuple (mem, error_message) (object, str)

free were equal to memory_used
used were equal to memory_free