AES encryption key defined in source code

Currently, the encryption key is barely and statically defined in source code [1] for 1_0_X, [2] for trunk.

This key must be moved to an external properties file (security.properties, for example).

Nice to have: random generation of this key during 'mvn archetype:generate'.

For 1_0_X: provide default to current key value [1] when not provided in security.properties.

[1] http://svn.apache.org/repos/asf/syncope/branches/1_0_X/core/src/main/java/org/apache/syncope/core/persistence/beans/user/SyncopeUser.java
[2] http://svn.apache.org/repos/asf/syncope/trunk/core/src/main/java/org/apache/syncope/core/util/PasswordEncoder.java