e2e-tests: Add JAVA_OPTS support to the framework

Unhardcode hostname and port values to start with, which typically vary
across test execution environments or deployments. This allows for less
json scenario data file duplication and proliferation.

Base this JAVA_OPTS solution on [1] below. If no JAVA_OPTS property|ies
gets defined in the scenario execution shell environment, use the
well-known default value(s) coded in. Document how to use JAVA_OPTS.

Adapt the showcase core CloneUsingBothProtocols and aggregated project
scenarios accordingly. Make them reuse core's amended GerritSimulation.

[1] https://gatling.io/docs/current/cookbook/passing_parameters

Change-Id: I45cf4b7ccf5cc51c8bd31fedbac99ae4bed053bb

e2e-tests/src/test/resources/data/com/google/gerrit/scenarios/CloneUsingBothProtocols.json

@@ -1,10 +1,10 @@
 [
   {
-    "url": "ssh://admin@localhost:29418/loadtest-repo",
+    "url": "ssh://admin@HOSTNAME:SSH_PORT/loadtest-repo",
     "cmd": "clone"
   },
   {
-    "url": "http://localhost:8080/loadtest-repo",
+    "url": "http://HOSTNAME:HTTP_PORT/loadtest-repo",
     "cmd": "clone"
   }
 ]

e2e-tests/src/test/resources/data/com/google/gerrit/scenarios/CreateProject.json

@@ -1,5 +1,5 @@
 [
   {
-    "url": "http://localhost:8080/a/projects/loadtest-repo"
+    "url": "http://HOSTNAME:HTTP_PORT/a/projects/loadtest-repo"
   }
 ]

e2e-tests/src/test/resources/data/com/google/gerrit/scenarios/DeleteProject.json

@@ -1,5 +1,5 @@
 [
   {
-    "url": "http://localhost:8080/a/projects/loadtest-repo/delete-project~delete"
+    "url": "http://HOSTNAME:HTTP_PORT/a/projects/loadtest-repo/delete-project~delete"
   }
 ]