Fix 04-create-server test to use a common prefix.

We need servers to dispose of for the 07-delete-server acceptance test.
Creating a bunch of servers in that test only to turn around and delete
them again will be unnecessarily time-consuming.  Instead, we'll use
ListServers() to discover a set of pre-existing servers to delete.
04-create-server already creates a batch, and will always execute before
07-delete-server (since 4 < 7).  Altering 04-create-server to put a tag
in the name of the servers it creates will allow 07-delete-server to
find them and delete them again.

The only exception is if any intervening tests fail.  You'll need to
clean up manually in that case.
Support delete server.
Merge pull request #44 from rackspace/delete-server

Delete server