Add external plugin tests to CI

Runs external plugin tests on fedora 29 with allowed failure.

At present this only tests against a branch, as there is tooling missing
in bst-plugins-experimental, and there is no tag which would be
compatible. In future there should be two targets, one testing plugins
master and one testing the last tag.

.gitlab-ci.yml

diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index ef31ad3..51e0f8c 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -18,7 +18,7 @@
   INTEGRATION_CACHE: "${CI_PROJECT_DIR}/cache/integration-cache"
   PYTEST_ARGS: "--color=yes --integration -n 2"
   TEST_COMMAND: "tox -- ${PYTEST_ARGS}"
-  EXTERNAL_TESTS_COMMAND: "tox -e py{35,36,37}-external -- ${PYTEST_ARGS}"
+  EXTERNAL_TESTS_COMMAND: "tox -e py37-plugins-master -- ${PYTEST_ARGS}"
   COVERAGE_PREFIX: "${CI_JOB_NAME}."
 
 
@@ -44,7 +44,6 @@
 
   # Run the tests as a simple user to test for permission issues
   - su buildstream -c "${TEST_COMMAND}"
-  - su buildstream -c "${EXTERNAL_TESTS_COMMAND}"
 
   after_script:
   except:
@@ -195,6 +194,18 @@
   variables:
     BST_FORCE_START_METHOD: "spawn"
 
+tests-external-plugins:
+  image: registry.gitlab.com/buildstream/buildstream-docker-images/testsuite-fedora:30-master-83237142
+  allow_failure: true
+  script:
+  - mkdir -p "${INTEGRATION_CACHE}"
+  - useradd -Um buildstream
+  - chown -R buildstream:buildstream .
+
+  # Run the tests as a simple user to test for permission issues
+  - su buildstream -c "${EXTERNAL_TESTS_COMMAND}"
+
+
 # Run type checkers
 mypy:
   stage: test