Added possibility to change perun's configuration files location on build or runtime.

 - added variable ${perun.conf.custom} which is not used by maven (but can be)
 - running jar/war with -Dperun.conf.custom=/home/user_name/.perunv3 will override default config from /etc/perunv3
 - changed way of wiring property file in perun-registrar
 - added definition of coreProperties to perun-core (not yet used to loading properties)