Add mysql jdbc dependency to maven-jetty-plugin

Pulls in the mysql jdbc driver as a dependency of the jetty maven plugin (and not the portal as a whole).

AC: Using this change, make sure jdbc.{user,password} are set in env.properties and the mysql jdbc jar is not on the classpath from elsewhere, and run "mvn clean jetty:run". It should start the development server without throwing the following exception:

PropertyAccessException 1: org.springframework.beans.MethodInvocationException: Property 'driverClassName' threw exception; nested exception is java.lang.IllegalStateException: Could not load JDBC driver class [com.mysql.jdbc.Driver]