gitlab-ci: add building on Fedora 31

And don't build Fedora 28 by default. It's aleady end of life.
But for now keep it, so it can be triggered manually.
18 jobs for th/fix-python-test in 90 minutes and 47 seconds (queued for 18 seconds)
Status Job ID Name Coverage
  Test
passed #657941
allowed to fail
checkpatch

00:01:11

passed #657947
allowed to fail manual
t_centos:7.5.1804

00:23:13

passed #657948
t_centos:7.6.1810

00:33:46

passed #657954
t_debian:10

00:18:34

passed #657953
t_debian:9

00:17:23

passed #657956
t_debian:sid

00:16:08

passed #657955
allowed to fail manual
t_debian:testing

00:18:17

passed #657942
allowed to fail manual
t_fedora:28

00:27:28

passed #657943
t_fedora:29

00:38:06

passed #657944
t_fedora:30

00:31:40

passed #657945
t_fedora:31

00:31:09

passed #657946
allowed to fail manual
t_fedora:rawhide

00:27:06

passed #657949
t_ubuntu:16.04

00:32:23

passed #657950
t_ubuntu:18.04

00:21:12

failed #659249
allowed to fail manual
t_ubuntu:devel

00:00:36

passed #657951
allowed to fail manual
t_ubuntu:rolling

00:51:35

failed #657952
allowed to fail manual
t_ubuntu:devel

00:26:18

 
  External
canceled https://desktopqe-jenkins.rhev-ci-vms.eng.rdu2.redhat.com/job/beaker-NetworkManager-gitlab-trigger-code-upstream/741/

20:57:17

 
Name Stage Failure
failed
t_ubuntu:devel Test
 * [new tag]         RELEASE_0_6_0               -> RELEASE_0_6_0
* [new tag] STABLE_0_3_4_RELEASE -> STABLE_0_3_4_RELEASE
* [new tag] STABLE_0_3_5_PRE_COMPLETION -> STABLE_0_3_5_PRE_COMPLETION
* [new tag] STABLE_0_3_5_PRE_SLEEP -> STABLE_0_3_5_PRE_SLEEP
* [new tag] STABLE_0_3_5_RELEASE -> STABLE_0_3_5_RELEASE
* [new tag] network_manager_initial -> network_manager_initial
Checking out 2bdb2496 as th/fix-python-test...
fatal: reference is not a tree: 2bdb2496975fce30e28e7d5ad14dc9d06721807a
ERROR: Job failed: exit code 1