added support for different linting depending on branch

The main reason for this commit is to provide different lint configurations
e.g. merge requests into master should follow slightly stricter linting rules.

.ci/.golangci.yml

+run:
+  timeout: 5m
+  issues-exit-code: 1
+output:
+  format: code-climate
+  print-issued-lines: true
+  print-linter-name: true
+  uniq-by-line: true
+  path-prefix: ""
+linters-settings:
+  gocyclo:
+    min-complexity: 15
+  golint:
+    min-confidence: 0.8
+linters:
+  disable-all: true
+  enable:
+    - gofmt
+    - golint
+    - gocyclo
+    - govet

.gitlab-ci.yml

 stages:
   - test
 
+code-quality-master:
+  image: golangci/golangci-lint:latest-alpine
+  tags:
+    - docker
+  stage: test
+  only:
+    - merge_requests
+  except:
+    variables:
+      - $CI_MERGE_REQUEST_TARGET_BRANCH_NAME != "master"
+  script:
+    - git config --global url."https://$GO_MODULES_USER:$GO_MODULES_ACCESS_TOKEN@code.fbi.h-da.de".insteadOf "https://code.fbi.h-da.de"
+    # writes golangci-lint output to gl-code-quality-report.json
+    - golangci-lint run --config .ci/.golangci-master.yml | tee gl-code-quality-report.json
+  artifacts:
+    reports:
+      codequality: gl-code-quality-report.json
+
+
 code-quality:
   image: golangci/golangci-lint:latest-alpine
   tags:
@@ -8,10 +27,13 @@ code-quality:
   stage: test
   only:
     - merge_requests
+  except:
+    variables:
+      - $CI_MERGE_REQUEST_TARGET_BRANCH_NAME == "master"
   script:
     - git config --global url."https://$GO_MODULES_USER:$GO_MODULES_ACCESS_TOKEN@code.fbi.h-da.de".insteadOf "https://code.fbi.h-da.de"
     # writes golangci-lint output to gl-code-quality-report.json
-    - golangci-lint run | tee gl-code-quality-report.json
+    - golangci-lint run --config .ci/.golangci.yml | tee gl-code-quality-report.json
   artifacts:
     reports:
       codequality: gl-code-quality-report.json