Changes

centos8-build: use nightly URL to invalidate cache

Fix copy paste mistake.

Change-Id: I1c5cfe782207af2a2e505fa30e77b9ea9fd143c5

centos8: don't download .repo file

Generate the .repo file on the fly instead of downloading it from the
OBS repository. Ensure the gpgkey gets downloaded via HTTPS. I'm about
to use centos8-obs-latest with an alternative repository, which does not
have the .repo file. Adjust other containers for consistency.

Related: SYS#5370
Related: https://download.opensuse.org/repositories/network:/osmocom:/latest/CentOS_8/network:osmocom:latest.repo
Related: https://download.opensuse.org/repositories/network:/osmocom:/nightly/CentOS_8/network:osmocom:nightly.repo
Change-Id: Ic9ffa79cfe5a74bdc59d5ddd505a9af7be574bf9

make: add OSMOCOM_REPO_PATH

Allow to change the path between OSMOCOM_REPO_MIRROR and
OSMOCOM_REPO_VERSION. While at it, tweak related comments (comment above
the variable as usually, replace "repo" wording with "feed" for the
latest/nightly variable as we usually refer to it as feed), and mention
OSMOCOM_REPO_* in README.md.

In order to be able to use a different mirror for testsuite and systems
under test, the testsuite related Dockerfiles (osmocom-bb-host-master,
debian-stretch-titan) are not using OSMOCOM_REPO_PATH. We could add a
OSMOCOM_REPO_TESTSUITE_PATH on demand, as mentioned in the next commit.

Related: SYS#5370
Change-Id: I8010a4d49d4620a5c47c2ff4f909a9aeeaad7b73

make: add OSMOCOM_REPO_TESTSUITE_MIRROR

Make it possible to set a different mirror for debian-stretch-titan than
for all other containers. 2021q1 doesn't have the eclipse-titan package
and it doesn't make sense to build it there.

I've thought about adding OSMOCOM_REPO_TESTSUITE_PATH and
OSMOCOM_REPO_TESTSUITE_VERSION too, but we don't have any use for these
right now. Let's add them later if we should need them.

Related: SYS#5370.
Change-Id: I7187473c2188a864b60b03789eff62e0a527404a

centos8-obs-latest: use OSMOCOM_REPO_VERSION

Related: SYS#5370
Change-Id: I5eb0d9943ac5c3ffe800774f1a91897fdf0ca0c3

osmo-*-latest: use OSMOCOM_REPO_VERSION

Related: SYS#5370
Change-Id: I237a296ea8a1e08bc31f351c5f42a59fe7438815