virttest.env_process: Include option to select encode video format.

Include support for option 'encode_video_format' to output
video with selected encoding format. Valid values are: webm, ogg or automatic.

Note that if this option is not setted, automatic detection will occurs:
try for webm or fallback to ogg.

Signed-off-by: Ruda Moura <rmoura@redhat.com>