Add function to execute QA commands with redirected output

Some QA commands generate non-trivial output: for a full QA, the gnt-*
list commands generate more than 300MB of data. To make the QA logs
more parseable by humans, we need support for redirecting output of
commands to a different place.

This patch adds a 'AssertRedirectedCommand' that does this,
redirecting the output to a (fixed) log file in the Ganeti log
directory.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>