multiplexer: Reintroduce command line options for filters.

* Reintroduce command line options `--filter-only` and `--filter-out` to control the global filters for multiplexing, both by the test runner and the multiplex plugin.
* Group multiplex command line options together in the group "multiplex" inside the test runner.

Examples:

<pre>
avocado run --help
... other options ...
multiplex arguments:
  -m MULTIPLEX_FILE, --multiplex-file MULTIPLEX_FILE
                        Path to an avocado multiplex (.yaml) file
  --filter-only [FILTER_ONLY [FILTER_ONLY ...]]
                        Filter only path(s) from multiplexing
  --filter-out [FILTER_OUT [FILTER_OUT ...]]
                        Filter out path(s) from multiplexing

avocado run footest --multiplex foo.yaml \
	--filter-only /root/env /root/os/linux \
        --filter-out /root/hw/virtio

avocado multiplex mux.yaml \
	--filter-only /root/os/win /root/hw/cpu/intel \
        --filter-out /root/hw/disk/virtio
Variants generated:
Variant 1:    /root/hw/cpu/intel, /root/hw/disk/scsi, /root/os/win/winxp, /root/env/debug
Variant 2:    /root/hw/cpu/intel, /root/hw/disk/scsi, /root/os/win/winxp, /root/env/prod
Variant 3:    /root/hw/cpu/intel, /root/hw/disk/scsi, /root/os/win/win7, /root/env/debug
Variant 4:    /root/hw/cpu/intel, /root/hw/disk/scsi, /root/os/win/win7, /root/env/prod
Variant 5:    /root/hw/cpu/intel, /root/hw/disk/scsi, /root/os/win/win8, /root/env/debug
Variant 6:    /root/hw/cpu/intel, /root/hw/disk/scsi, /root/os/win/win8, /root/env/prod
</pre>

Signed-off-by: Ruda Moura <rmoura@redhat.com>