Allow custom server backend encoding
    
    Server framework does not support encoding of native Python type
    values stored in Param classes and sub-classes. When backend (LDAP)
    value encoding differs from Python type value representation user
    has to has to hard-code the encoders in his processing.
    
    This patch introduces a method Param.encode which is used in server
    context to encode native Python Param values. The new encode method
    is used for Bool parameter to convert native Python bool type value
    (True, False) to LDAP value ("TRUE", "FALSE").
    
    https://fedorahosted.org/freeipa/ticket/2039