gitlab-ci: combine ubuntu/centos install scripts with their debian/fedora counterparts

Ubuntu/Debian and CentOS/Fedora are sufficiently similar that it's
better that we have only one variant of ".gitlab-ci/*-install.sh"
and "contrib/*/REQUIRED_PACKAGES".

This was already the case, however, we used to symlink
".gitlab-ci/centos-install.sh" to "fedora-install.sh". That
worked, but it didn't scale very well. For example, if we would follow
that pattern, we would also need a symlink "contrib/centos/REQUIRED_PACKAGES"
Or should "contrib/centos" symlink to "contrib/fedora"? That seems even
more wrong.

We already had the "distro.base_type" variable for that. Make use of
that also for the install script.

.gitlab-ci.yml

@@ -41,15 +41,15 @@ variables:
   # changing these will force rebuilding the associated image
   # Note: these tags have no meaning and are not tied to a particular
   # libinput version
-  FEDORA_TAG:  '2020-11-17.0-62ff0634a962'
+  FEDORA_TAG:  '2020-11-17.0-b6f225d87807'
-  UBUNTU_TAG:  '2020-11-17.0-a7a0eae3a046'
+  UBUNTU_TAG:  '2020-11-17.0-e2797fbbfc3a'
-  DEBIAN_TAG:  '2020-11-17.0-f3fdbff29bf1'
+  DEBIAN_TAG:  '2020-11-17.0-e2797fbbfc3a'
-  CENTOS_TAG:  '2020-11-17.0-212105f711c8'
+  CENTOS_TAG:  '2020-11-17.0-b6f225d87807'
 
   FEDORA_EXEC: 'bash .gitlab-ci/fedora-install.sh'
-  UBUNTU_EXEC: 'bash .gitlab-ci/ubuntu-install.sh'
+  UBUNTU_EXEC: 'bash .gitlab-ci/debian-install.sh'
   DEBIAN_EXEC: 'bash .gitlab-ci/debian-install.sh'
-  CENTOS_EXEC: 'bash .gitlab-ci/centos-install.sh'
+  CENTOS_EXEC: 'bash .gitlab-ci/fedora-install.sh'
 
 .nm_artifacts:
   variables:

.gitlab-ci/centos-install.sh

@@ -1 +0,0 @@
-fedora-install.sh

.gitlab-ci/ci.template

@@ -37,13 +37,13 @@ variables:
   {{"%-13s"| format(distro.name.upper() + '_TAG:')}}'{{distro.tag}}-{{
     (ci_fairy.hashfiles('./.gitlab-ci/config.yml',
                         './.gitlab-ci/ci.template',
-                        './.gitlab-ci/' + distro.name + '-install.sh',
+                        './.gitlab-ci/' + distro.base_type + '-install.sh',
                         './contrib/' + distro.base_type + '/REQUIRED_PACKAGES'))[0:12]
   }}'
 {% endfor %}
 
 {% for distro in distributions %}
-  {{"%-13s"| format(distro.name.upper() + '_EXEC:')}}'bash .gitlab-ci/{{distro.name}}-install.sh'
+  {{"%-13s"| format(distro.name.upper() + '_EXEC:')}}'bash .gitlab-ci/{{distro.base_type}}-install.sh'
 {% endfor %}
 
 .nm_artifacts:

.gitlab-ci/ubuntu-install.sh

@@ -1 +0,0 @@
-debian-install.sh