gitlab-ci: Use templates from wayland/ci-templates

These are already used by xserver, Mesa and some other projects.

Current Debian testing brings e.g. GCC 8.3.0 and clang 7.0.1.
13 jobs for gitlab-ci-templates in 11 minutes and 34 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Docker Image
passed #467081
debian-testing

00:08:39

failed #467069
debian-testing

00:00:54

 
  Build
passed #467072
xserver-1.13

00:00:28

passed #467073
xserver-1.14

00:00:35

passed #467074
xserver-1.15

00:00:33

passed #467075
xserver-1.16

00:00:28

passed #467076
xserver-1.17

00:00:29

passed #467077
xserver-1.18

00:00:48

passed #467078
xserver-1.18-clang

00:00:34

passed #467079
xserver-1.19

00:01:07

passed #467080
xserver-1.19-clang

00:01:12

passed #467070
xserver-1.20

00:00:56

passed #467071
xserver-1.20-clang

00:00:50