Put db requirements to extra requirements

https://www.python.org/dev/peps/pep-0426/#extras-optional-dependencies

Python is a great language and it support extra requirements. In our
case, it can be used for listing database requirements.

This patch adds two separate extra requirements:
  * psycopg2 as for Postgres database backend
  * PyMySQL as for MySQL database backend

To install Rally with a specific database backend you need just to do
execute pip in right way:
    pip install rally[postgres]
  or
    pip install rally[mysql]

Change-Id: Ibf816758fc093a65b94e6a2b2c273a073275b393

setup.cfg

@@ -39,6 +39,12 @@ oslo.config.opts =
 setup-hooks =
     pbr.hooks.setup_hook
 
+[extras]
+mysql =
+  PyMySQL>=0.7.6  # MIT
+postgres =
+  psycopg2>=2.5  # LGPL/ZPL
+
 [build_sphinx]
 all_files = 1
 build-dir = doc/build

tox.ini

@@ -4,6 +4,7 @@ skipsdist = True
 envlist = py35,py34,py27,pep8
 
 [testenv]
+extras = {env:RALLY_EXTRAS:}
 setenv = VIRTUAL_ENV={envdir}
          HOME={homedir}
          LANG=en_US.UTF-8