Simplify QA commands

Currently, 95% of the QA commands are executed in the same way: on the
master, based on a command list and with expectancies for succes:

    AssertEqual(StartSSH(master['primary'],
                         utils.ShellQuoteArgs(cmd)).wait(), 0)

The rest 5% are variations on this theme (maybe the command needs to
fail, or the node is different, etc.). Based on this, we can simplify
the code significantly if we abstract the common theme into a new
AssertCommand() function. This saves ~250 lines of code in the QA suite,
around 8% of the entire QA code size.

Additionally, the output was very cryptic before (the famous "QA error:
1 != 0" messages), whereas now we show a clear error message (node,
command, exit code and failure mode).

The patch replaces single quotes with double quotes in all the parts of
the code that I touch; let me know if that's not OK…

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