Parameterize the jira::dburl variable

This is so $dburl can be overriden as a workaround for the following bug in Mysql Connector/J 5.1.16:
https://confluence.atlassian.com/display/JIRAKB/%27NullPointerException+at+com.mysql.jdbc.ConnectionImpl.getServerCharacterEncoding%27+Error+When+Using+MySQL+Driver+v5.1.16

Specifically, I've found that setting jira::dburl to "'jdbc:mysql://127.0.0.1:3305/jira?useUnicode=true&amp;characterEncoding=UTF8" works.