Have choice between MySQL-python and PyMySQL.

The latter is a pure-Python implementation which has benefits when using
eventlet and a single process to access the DB. For Crowbar, this
can translate into better performance when only one nova-conductor is
present (the default).
Merge pull request #12 from saschpe/release/pebbles/master-pymysql

Have choice between MySQL-python and PyMySQL.
Merge pull request #3 from crowbar/release/pebbles/master

pull last changes