MXS-862: Add authenticator options and instances

Authenticators now have a similar mechanism to the `router_options`
parameter which enables configurable authentication.

The authenticators also have a new initialize entry point which is similar
to the createInstance entry point of the filters and routers. The value of
`authenticator_options` is passed as a parameter to this function. The
return vaulue of the `initialize` entry point is passed to the `create`
entry point.

server/modules/authenticator/gssapi_auth_common.c

@@ -14,7 +14,7 @@
 #include "gssapi_auth.h"
 #include <maxscale/alloc.h>
 
-void* gssapi_auth_alloc()
+void* gssapi_auth_alloc(void *instance)
 {
     gssapi_auth_t* rval = MXS_MALLOC(sizeof(gssapi_auth_t));