soften statsd version requirement to allow v2

We used to pin statsd to version 1.0.0.  James E. Blair has written a
few tests that now let us allow the current v2.0.1 version.  To prevent
unexpected upgrades, we disallow v3.0.0.

Zuul scheduler only do very simple statsd operations. Testing with
statsd 2.0.1:

I have set up a basic listener:

 listener$ netcat -l 127.0.0.1 -p 666 --udp

Then wrote the python script:

  import extras
  statsd = extras.try_import('statsd.statsd')

  statsd.incr('zuul.counter')
  statsd.timing('zuul.timer', 1234)
  statsd.timing('zuul.timer', 1)
  statsd.gaug:('zuul.gauge', 1234)

Running it as:
  STATSD_HOST='127.0.0.1' STATSD_PORT=666 python statsd-test.py

The listener gets:

  zuul.counter:1|czuul.timer:1234|mszuul.timer:1|mszuul.gauge:1234|g

Sounds legit enough.

Fix bug #1169599 'Zuul should depends on statsd 2 or later'

Change-Id: I41b797d606d55eeda300e673b05c2ae4442ce029
Reviewed-on: https://review.openstack.org/27008
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins

tools/pip-requires

@@ -7,5 +7,5 @@ GitPython>=0.3.2.RC1
 lockfile
 python-daemon
 extras
-statsd==1.0.0
+statsd>=1.0.0,<3.0
 voluptuous>=0.6,<0.7