Change the LoadTestAction to use /_dr/epptool

The load tests used to directly build EPP, but that becomes
problematic for an upcoming CL that refactors a lot of the
EPP flow code. Instead, use the existing tool endpoint
(conveniently, LoadTestAction is already in the tools module).
This required changing the EppToolServlet to get its
xml from a param rather than from the request payload,
since task queues won't allow both a payload and request
params on the same task.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=124351878

javatests/google/registry/tools/DomainApplicationInfoCommandTest.java

@@ -26,7 +26,7 @@ public class DomainApplicationInfoCommandTest
   public void testSuccess() throws Exception {
     runCommandForced("--client=NewRegistrar", "--domain_name=example.tld",
         "--phase=landrush", "--id=123");
-    eppVerifier().verifySent("testdata/domain_info_landrush.xml");
+    eppVerifier().verifySent("domain_info_landrush.xml");
   }
 
   @Test
@@ -34,7 +34,7 @@ public class DomainApplicationInfoCommandTest
     // Sunrush: phase=sunrise, subphase=landrush
     runCommandForced("--client=NewRegistrar", "--domain_name=example.tld",
         "--phase=sunrush", "--id=123");
-    eppVerifier().verifySent("testdata/domain_info_sunrush.xml");
+    eppVerifier().verifySent("domain_info_sunrush.xml");
   }
 
   @Test