virttest.env_process: Include option to enforce video format.

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

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

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