fix handling of arguments when main() is called without arguments
fix handling of arguments when main() is called without arguments

As mentioned in comment https://github.com/ordergroove/check_mariadb_slaves/commit/faa3c2dba77bd9e853b6277723a93b6fbfd5ba63#commitcomment-15392749
when calling check_mariadb_slaves.main() without arguments, the list got from sys.argv
contains the script name as first element. This commit restructures the handling of
default argument list slightly, in order to fix this issue and pass the added test
in the previous commit.

ordergroove/check_mariadb_slaves