copr_test: fixes and improvements for resiliency

[smoser]

The biggest fix here is retrying the download of yum data up to 10 times
with a backoff. This is the fix we employed in
cloud-init/tools/run-centos and have been successful with it. This retry is
done in an added 'install_packages' function.

• drop unused function inside_as
• inside(): add LANG=C LC_ALL=C to avoid tools inside complaining about
  locale. (yum would complain: Failed to set locale, defaulting to C)
• inside(): debug 1 with the command being run.
• debug(): prefix output with '::' so it is easily found among noise.
• wait_for_network: do not redirect 'inside's stderr, only the command
  being run. This allows us to see the debug statement from inside.
• do not use 'sh -c' when not needed.
• fix '--project' and '--group' options to take an option.
• fix '-v' to not take an option (making -v actually work).