use python_pip to install the same recent txAMQP lib for all platforms

See https://github.com/hw-cookbooks/graphite/issues/27 .

Validated on all vagrant VMs ('cept for centos 5.8 which I disabled).

Tested this by also installing rabbitmq cookbook and sending metrics from a ruby-amqp test client on my local machine. On Ubuntu 10.04, the rabbitmq version is old so ruby-amqp failed to connect (but graphite did ;-)

Would have to see if/how this can be added to tests - along with testing a metric via TCP.