add DynamicUserAuthenticator for VFS access

1/- add setter for ftpControllEncoding in VfsAccessImpl defaulting to "UTF-8"

2/- add a VFS DynamicUserAuthenticator wrapping call to UserAuthenticatorService.getUserPassword. 
This authenticator is used with CasUserAuthenticatorService so when the vfs back-end require auth; a new proxyTicket is requested.

VFS back-end (at least FTP back-end) handle reconnecting back-end when an IOException is thrown.

Ftp server always have idle timeout; so when this timeout is reached; the back-end must re-authenticate. 
For CasUserAuthenticatorService, a new serviceTicket must be issued to successfully authenticate (unless caching ticket server side). 

Common pitfall : userParameters CASReceipt must be set