Fix default-storage-engine default value

Without this commit, puppetlabs-mysql breaks mysql configuration by inserting a
dummy default-storage-engine directive. This will prevent mysql daemon from
starting up.

For example:

notice: /Stage[main]/Mysql::Config/File[/etc/mysql/my.cnf]/content:
--- /etc/mysql/my.cnf   2013-04-25 21:36:25.000000000 +0000
+++ /tmp/puppet-file20130425-12937-10tcf50-0    2013-04-25 21:42:49.000000000 +0000
@@ -29,6 +29,8 @@
 expire_logs_days   = 10
 max_binlog_size    = 100M

+default-storage-engine = undef
+

 [mysqldump]