Change ca certificate path config key. (#195233)

towards: elastic/response-ops-team#209

This PR changes ca path key in the config .

(cherry picked from commit e1cf8d5)

x-pack/plugins/actions/server/config.ts

@@ -147,7 +147,7 @@ export const configSchema = schema.object({
   ),
   usage: schema.maybe(
     schema.object({
-      cert: schema.maybe(
+      ca: schema.maybe(
         schema.object({
           path: schema.string(),
         })