Make AuthenticationManager pluggable

This improvement forms part of the proposal I made to the Qpid Developer List on 2011-05-16 in the posting entitled "Java Broker: Pluggable Authentication Managers".

The existing AuthenticationManager and configuration classes will be refactored to allow a user to choose an implementation of AuthenticationManager through new elements in the config.xml.

Existing use-cases (that is authentication against the etc/passwd file) will be maintained by plugging-in the PrincipalDatabaseAuthenticationManager implementation. Future implementations of AuthenticationManager could then allow authentication to be delegated to an external system such as an LDAP Directory.

This change will alter the format of the config.xml file. Existing users of <=0-12 will need to edit their config.xml when upgrading. Documentation will be updated to guide upgraders and helpful error messages will gently reject older configure files.

See also:

https://cwiki.apache.org/confluence/display/qpid/Java+Pluggable+Authentication+Managers