QA: stop logging SSH arguments for each invocation

Currently, this is how the QA log looks like:

Command: ssh -oEscapeChar=none -oBatchMode=yes -l root -t -oStrictHostKeyChecking=yes -oClearAllForwardings=yes -oForwardAgent=yes -oControlPath=/tmp/ganeti-qa-multiplexer.DMzkuH -oControlMaster=no node14 exit
Command: ssh -oEscapeChar=none -oBatchMode=yes -l root -t -oStrictHostKeyChecking=yes -oClearAllForwardings=yes -oForwardAgent=yes node8 exit
Command: ssh -oEscapeChar=none -oBatchMode=yes -l root -t -oStrictHostKeyChecking=yes -oClearAllForwardings=yes -oForwardAgent=yes node18 exit

And these arguments are repeated over and over. This patch proposes to
log once the arguments, at the beginning of the QA (so that the SSH
commands can be reproduced, if needed) and then drop them from the
log. The new output looks like:

SSH command for primary node: ssh -oEscapeChar=none -oBatchMode=yes -lroot -oStrictHostKeyChecking=yes -oClearAllForwardings=yes -oForwardAgent=yes -oControlPath=/tmp/ganeti-qa-multiplexer.24lgrK -oControlMaster=no node14
SSH command for other nodes: ssh -oEscapeChar=none -oBatchMode=yes -lroot -oStrictHostKeyChecking=yes -oClearAllForwardings=yes -oForwardAgent=yes NODE
(the above are the informational messages about parameters, then)
Command: ssh node14 exit
Command: ssh node8 exit
Command: ssh node18 exit

This makes the QA log much more readable, by dropping unneeded clutter
(look how long the original lines were), and hopefully also a bit
smaller (current QA log is > 100MB of text output).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Bernardo Dal Seno <bdalseno@google.com>