This tries to solve a failure in building rpms, and consists of two
fixes.

1. Looking into cri-o CI, this error seems common:

	http://vault.centos.org/centos/7/os/Source/repodata/repomd.xml: [Errno 14] curl#7 - "Failed to connect to 2600:1f16:c1:5e03:a992:8ee1:99d2:6f8: Network is unreachable"
	http://vault.centos.org/centos/7/os/Source/repodata/repomd.xml: [Errno 14] curl#7 - "Failed to connect to 2600:1f16:c1:5e03:a992:8ee1:99d2:6f8: Network is unreachable"
	http://vault.centos.org/centos/7/os/Source/repodata/repomd.xml: [Errno 14] HTTPS Error 301 - Moved Permanently
	[ERROR] hack/build-rpms.sh:31: `yum-builddep -y "${OS_RPM_SPECFILE}"` exited with status 1.

Apparently, vault.centos.org resolves to both IPv4 and IPv6 addresses,
and one of the addresses is used (randomly, I presume). If the node
does not have IPv6 connectivity (as it seems to be the case), as a
result we have 50% chance of failure.

Work around this by asking yum to only use IPv4 for address resolving.

2. vault.centos.org is super slow, resulting in numerous timeouts,
   often fatal:

	http://vault.centos.org/centos/7/os/Source/repodata/repomd.xml: [Errno 12] Timeout on http://vault.centos.org/centos/7/os/Source/repodata/repomd.xml: (28, 'Operation too slow. Less than 1000 bytes/sec transferred the last 30 seconds')
	http://vault.centos.org/centos/7/os/Source/repodata/repomd.xml: [Errno 12] Timeout on http://vault.centos.org/centos/7/os/Source/repodata/repomd.xml: (28, 'Connection timed out after 30001 milliseconds')
	http://vault.centos.org/centos/7/os/Source/repodata/repomd.xml: [Errno 12] Timeout on http://vault.centos.org/centos/7/os/Source/repodata/repomd.xml: (28, 'Connection timed out after 30001 milliseconds')
	http://vault.centos.org/centos/7/os/Source/repodata/repomd.xml: [Errno 12] Timeout on http://vault.centos.org/centos/7/os/Source/repodata/repomd.xml: (28, 'Connection timed out after 30001 milliseconds')
	[ERROR] hack/build-rpms.sh:38: `yum-builddep -y "${OS_RPM_SPECFILE}"` exited with status 1.

Let's use mirrors.kernel.org instead.

Signed-off-by: Kir Kolyshkin <[email protected]>