repositories: Fix percona pinning on debian

percona-toolkit package is also present in debian repository. Have a pinning
at 100 for percona repository is not enough to get latest release.
We need to have a pinning value > 500 (default debian repository).

Signed-off-by: Dimitri Savineau <dimitri.savineau@enovance.com>
Merge pull request #112 from enovance/fix-percona-pinning

repositories: Fix percona pinning on debian