gitlab-ci: run tests on extra distributions only manually

For the moment, we use docker images from dockerhub, which require
a lot of extra overhead to prepare and install the test environment.
This should be improved, by using more suitable container images.

Anyway, for now to alleviate the pressure on the freedesktop gitlab
infrastructure, disable most test to only run manually.

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/issues/241#note_282521

.gitlab-ci.yml

@@ -90,6 +90,7 @@ t_fedora:29:
   <<: *fedora_install
   image: fedora:29
   <<: *do_build
+  when: manual
 
 t_fedora:30:
   <<: *fedora_install
@@ -108,6 +109,7 @@ t_fedora:31:
   <<: *fedora_install
   image: fedora:31
   <<: *do_build
+  when: manual
 
 t_fedora:rawhide:
   <<: *fedora_install
@@ -126,16 +128,19 @@ t_centos:7.6.1810:
   <<: *fedora_install
   image: centos:7.6.1810
   <<: *do_build
+  when: manual
 
 t_ubuntu:16.04:
   <<: *debian_install
   image: ubuntu:16.04
   <<: *do_build
+  when: manual
 
 t_ubuntu:18.04:
   <<: *debian_install
   image: ubuntu:18.04
   <<: *do_build
+  when: manual
 
 t_ubuntu:rolling:
   <<: *debian_install
@@ -153,11 +158,13 @@ t_debian:9:
   <<: *debian_install
   image: debian:stretch
   <<: *do_build
+  when: manual
 
 t_debian:10:
   <<: *debian_install
   image: debian:stretch
   <<: *do_build
+  when: manual
 
 t_debian:testing:
   <<: *debian_install
@@ -169,6 +176,7 @@ t_debian:sid:
   <<: *debian_install
   image: debian:sid
   <<: *do_build
+  when: manual
 
 pages:
   stage: deploy