Use command line --config to read a custom configuration from a file.
Note this option is for the avocado application, so it has
effect for all subcommands.

Example:

$ avocado --config /var/tmp/avocado.conf config
Config files read (in order):
/home/rmoura/Work/avocado/etc/avocado/avocado.conf
/home/rmoura/Work/avocado/etc/avocado/conf.d/gdb.conf
/var/tmp/avocado.conf
...

Also, update man-page about the new option --config.
Signed-off-by: NRudá Moura <rmoura@redhat.com>