Without this change, operating system elements that use the
zypper-minimal element always must use download.opensuse.org as their
repository source. This change makes ZYPPER_REPOS overrideable, which
allows the user to create custom operating system elements that can use
private repositories as their source for base packages. For example,
with only this change, it is possible to create a sles-minimal element
that generates a SLE 15 SP1 image just by overriding DIB_ZYPPER_REPOS
and DIB_OPENSUSE_PATTERNS.
Change-Id: I46e40fbe4408d4204056a27b182b21213f1176ff
ad67aa213b