Add CentOS 6/7 to CI

Modify .gitlab-ci.yml so that every CI pipeline also builds and tests
BIND on CentOS versions 6 and 7.  Use --disable-warn-error on CentOS 6
since it uses GCC 4.4.7 which suffers from bugs causing bogus warnings
to be generated, e.g.:

    sigs_test.c: In function 'compare_tuples':
    sigs_test.c:75: warning: declaration of 'index' shadows a global declaration
    /usr/include/string.h:489: warning: shadowed declaration is here
    sigs_test.c: In function 'updatesigs_test':
    sigs_test.c:193: warning: declaration of 'index' shadows a global declaration
    /usr/include/string.h:489: warning: shadowed declaration is here
16 jobs for 365-add-centos-to-ci in 9 minutes and 34 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Precheck
passed #24354
amd64 docker linux
precheck:debian:sid:amd64

00:00:13

 
  Build
passed #24355
amd64 docker linux
build:centos:centos6:amd64

00:02:52

passed #24356
amd64 docker linux
build:centos:centos7:amd64

00:03:03

passed #24357
amd64 docker linux
build:clang:debian:sid:amd64

00:03:38

passed #24359
i386 docker linux
build:clang:debian:sid:i386

00:03:22

passed #24358
amd64 docker linux
build:debian:sid:amd64

00:02:28

passed #24360
i386 docker linux
build:debian:sid:i386

00:02:34

 
  Test
passed #24366
amd64 docker linux
systemtest:centos:centos6:amd64

00:04:43

passed #24367
amd64 docker linux
systemtest:centos:centos7:amd64

00:04:43

passed #24368
amd64 docker linux
systemtest:debian:sid:amd64

00:04:45

passed #24369
i386 docker linux
systemtest:debian:sid:i386

00:04:48

passed #24361
amd64 docker linux
unittest:centos:centos6:amd64

00:00:54

passed #24362
amd64 docker linux
unittest:centos:centos7:amd64

00:00:56

passed #24364
amd64 docker linux
unittest:clang:debian:sid:amd64

00:00:49

passed #24363
amd64 docker linux
unittest:debian:sid:amd64

00:00:50

passed #24365
i386 docker linux
unittest:debian:sid:i386

00:00:43