man: Explain --silent effect

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.job: Use sequential numbers for variant tags, do not encode names

Basically, trying to encode information of the variants themselves
in the tag name is a lost battle, since:

1) We need unique test ids, and sometimes only the last part of
   the name of each variant combined is not going to cut it
2) Coding the entire path of each variant combined would create
   in many cases enormous tags and the point would be lost

So go back and use sequential numbers that match the output
of the multiplex plugin. This way it'll be easy to correlate
variants and tag numbers.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.plugins.multiplexer: Reduce message priority for better output

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
man: Explain --job-log-level option

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>