proxy: Alocate auth tokens in struct authtok_conv
    
    Struct sss_auth_token became opaque in commit
    9acfb09f7969a69f58bd45c856b01700541853ca.
    All ocasions of "struct sss_auth_token" was replaced with pointer to this
    struct, but proper initialization of auth_tokens was missing
    in struct authtok_conv.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2046