Externalized property overrider for OpenSDI-Manager

Add a new property overrider to be used as system property link to a file.

For example if you want to run jetty from code with a externalized property:

`*-web-extension$ mvn jetty:run -DOPENSDI_CONFIG_FILE=/tmp/opensdi-config.properties`

uses a externalized properties file located on `/tmp/opensdi-config.properties`.

You can configure it for tomcat instances adding this property to JAVA_OPTS.
Merge pull request #36 from alediator/property_ovr

Externalized property overrider for OpenSDI-Manager