automation: skip some branches in gitlab CI

Ignore branches which are always fast-forwarded to staging* branches.
List of filters taken from Travis CI setup.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Doug Goldstein <cardoe@cardoe.com>

.gitlab-ci.yml

@@ -11,6 +11,11 @@ stages:
       - xen/.config
       - '*.log'
     when: always
+  except:
+    - master
+    - smoke
+    - /^coverity-tested\/.*/
+    - /^stable-.*/
 
 .gcc-tmpl:
   variabes: &gcc