testapp & pom: add testapp file check & change required maven version

This patch is more on the customer-facing side, i.e. to increase usability for potential mupd8 users, and keep them happy.

* The "testapp.sh stop" routine adds a check of pid file existence, so it prints more concise message when the mupd8 processes are not found, instead of printing kill command usage. 

* The Maven required version is lowered to 2.2.0 to accommodate more audience running different versions of this build tool.  I went through a full cycle of build and install and run the product and make sure it's okay on 2.2.1.