dmeventd_thin: enable support for external command

With this commit we start to support configurable action
from thin-pool monitoring via  'dmeventd/thin_command'

WHATS_NEW_DM

@@ -1,5 +1,6 @@
 Version 1.02.138 - 
 =====================================
+  Support configurable command executed from dmeventd thin plugin.
   Support new R|r human readable units output format.
   Thin dmeventd plugin reacts faster on lvextend failure path with umount.
   Add dm_stats_bind_from_fd() to bind a stats handle from a file descriptor.

daemons/dmeventd/plugins/thin/dmeventd_thin.c

@@ -357,8 +357,7 @@ int register_device(const char *device,
 
 	if (!dmeventd_lvm2_command(state->mem, state->cmd_str,
 				   sizeof(state->cmd_str),
-				   "lvextend --use-policies",
-				   device)) {
+				   "_dmeventd_thin_command", device)) {
 		dmeventd_lvm2_exit_with_pool(state);
 		goto_bad;
 	}