Hardcode MySql version to 5.5.29

- The default in brooklyn 0.5.0-M1 is 5.5.28 which is no longer
  available for download from the official MySql site
- Not nice to hard-code a version here, but required to make it work!
  We should remove this config when we next bump the brooklyn version.
Merge pull request #4 from aledsage/fix/MySql-version

Hardcode MySql version to 5.5.29