Use LONGBLOB for certs in MySQL.

BLOB in MySQL can only store values up to 65KB.  Use LONGBLOB
instead which holds up to 4GB.  See
http://dev.mysql.com/doc/refman/5.5/en/storage-requirements.html
Merge pull request #505 from candlepin/awood/mysql-longblobs

Use LONGBLOB for certs in MySQL.