CentOS keeps non-x86 architectures in /altarch/ directory (contrary to
/centos/ one for x86-64). We have aarch64 (arm64) machines in infra and
they fail due to lack of CentOS altarch mirror.
List of wanted alternative architectures is controlled by ALTARCHS
variable (aarch64 and ppc64le enabled). As CentOS has several other
architectures too they are listed in ALTARCHS_IGNORED so we do not fetch
them.
Current CentOS mirror lands in same /mirror/centos/7/ directory. Altarch
mirrors goes to /mirror/centos/altarch/7/ one.
Change-Id: I0e110ef51c8ed301fd8280ae7fc039e3b01db92c
ac5acbca92