Fix tox's insane pip install command.

Override tox's pip install command so that prereleases (alphas, betas,
broken things) aren't installed by default. Instead only install release
versions unless a prerelease version is specifically provided. Do this
by overriding the tox install command value.

This fixes doc builds that use `tox -evenv python setup.py
build_sphinx`.

Also, remove the gearman servers from the test configuration (since there
is not yet any support for it).

Change-Id: I31913420adcb48866d3996f2dd3b605c55acce2e

nodepool/tests/fixtures/node.yaml

@@ -9,8 +9,8 @@ cron:
 zmq-publishers:
   - tcp://localhost:8881
 
-gearman-servers:
+#gearman-servers:
-  - host: localhost
+#  - host: localhost
 
 labels:
   - name: fake-label

nodepool/tests/fixtures/subnodes.yaml

@@ -9,8 +9,8 @@ cron:
 zmq-publishers:
   - tcp://localhost:8881
 
-gearman-servers:
+#gearman-servers:
-  - host: localhost
+#  - host: localhost
 
 labels:
   - name: fake-label

tox.ini

@@ -6,6 +6,7 @@ envlist = pep8, py27
 setenv = STATSD_HOST=localhost
          STATSD_PORT=8125
          VIRTUAL_ENV={envdir}
+install_command = pip install {opts} {packages}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands =