CLOUDSTACK-6360: Fix JDK paths in ubuntu packaging init scripts, don't use 1.6

Since we've agreed to use JDK/JRE 1.7, this enforces that for Ubuntu builds
- this fix remove usage of 1.6 paths in JDK_DIR for cloud-{agent, management, usage}.
- adds oracle jdk 1.7 path (in case a user is using that)
- adds mysql-connector-java path to CLASSPATH for usage server
- adds libmysql-java pkg dependency (tested and available for precise and trusty)

Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>