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:

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

Signed-off-by: Ruda Moura <rmoura@redhat.com>
Merge pull request #214 from ruda/multiplex_filters_command_line_options

multiplexer: Reintroduce command line options for filters.