Add QA_CAN_TEST_GIT_PROTOCOL_V2

Add QA_CAN_TEST_GIT_PROTOCOL_V2 so tests that require Git protocol v2 can be skipped in environments that don't support it.

Required so that the test added in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22548 can be skipped in nightly and package-and-qa pipelines until they can support Git protocol v2.

docs/run_qa_against_gdk.md

@@ -4,11 +4,11 @@ To run the `Test::Instance::Any` scenario against your local GDK, you'll need to
 make a few changes to your `gdk/gitlab/config/gitlab.yml` file.
 
 1. Retrieve your current local IP with `ifconfig`, e.g. `192.168.0.12`.
-1. Edit `gdk/gitlab/config/gitlab.yml` and replace `host: localhost` with 
+1. Edit `gdk/gitlab/config/gitlab.yml` and replace `host: localhost` with
    `host: 192.168.0.12`.
-1. Also replace `ssh_host: localhost` (found under `gitlab_shell`) with 
+1. Also replace `ssh_host: localhost` (found under `gitlab_shell`) with
    `ssh_host: 192.168.0.12`.
-1. Enable the `sshd` service by uncommenting the relevant line in your 
+1. Enable the `sshd` service by uncommenting the relevant line in your
    `Procfile`.
 1. Edit `openssh/sshd_config` and set `ListenAddress` to `192.168.0.12`.
 1. Restart your GDK
@@ -29,6 +29,13 @@ make a few changes to your `gdk/gitlab/config/gitlab.yml` file.
   $ bin/qa Test::Instance::Any gitlab/gitlab-ce:your-custom-tag http://192.168.0.12:3000 -- qa/specs/features/repository/protected_branches_spec.rb:30
   ```
 
+If you want to run tests that require Git protocol v2 you'll need to add the
+following to `openssh/sshd_config`:
+
+```
+AcceptEnv GIT_PROTOCOL
+```
+
 ## Run Geo QA tests against your Geo GDK setup
 
 Run from the `gdk-ee/gitlab/qa` directory with GDK primary and secondary running: