Add --times to e2e.go

Make each test available for execution --times times. Acts like a
multi-deck shoe. (Otherwise this is easily scriptable outside the
e2e.go command).

Useful for "I want to walk away for a few hours leaving some end-to-ends
running", until we have more stress tests. Also useful for detecting
flakes.

Note that I didn't tackle reporting, which is still pretty rudimentary
for e2e.go. Now it looks like:

2014/12/08 15:32:58 Passed tests: [basic.sh basic.sh basic.sh certs.sh certs.sh certs.sh goe2e.sh goe2e.sh goe2e.sh guestbook.sh guestbook.sh guestbook.sh monitoring.sh monitoring.sh monitoring.sh pd.sh pd.sh pd.sh private.sh private.sh private.sh services.sh services.sh services.sh update.sh update.sh update.sh]
2014/12/08 15:32:58 Failed tests: []

Easy enough to post-process.
(nit) does this really need to be a struct? why not just ```type ResultsByTest map[string]TestResult```

... and then you can construct it with the shortcut ```foo := ResultsByTest{}``` (or ```make(ResultsByTest)```) which is nicer reading and less typing...