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.
It's not strictly invalid, just silly.

 TAP version 13
# 2014/12/11 11:58:10 Running in background: watchEvents
# 2014/12/11 11:58:10 Running: get status
# 2014/12/11 11:58:13 Running tests matching fake.sh shuffled with seed 0x6655e250: []
1..0
# watchEvents stdout: |
#   Project: artful-fortress-786 (autodetected from gcloud config)
#
# 2014/12/11 11:58:13 Passed tests:
# 2014/12/11 11:58:13 Flaky tests:
# 2014/12/11 11:58:13 Failed tests:
# 2014/12/11 11:58:13 Success!