Add --skip-nvr-check to the scratch-build command
When building in EPEL7 for the different target system, the build
sometimes fails with unknown or missing macro/tag. This allows
the packager to skip this rpm fail and continue with
'--skip-nvr-check' argument. Previously it was allowed for
subcommands build and chain-build. Now, argument parsing code could
be moved to the common method.
JIRA: RHELCMP-593
Resolves: rhbz#1671012
Signed-off-by: Ondrej Nosek <onosek@redhat.com>