Change standard config path and add override config

The standard config has moved to /usr/share/containers/ per
discussion. An override configuration file is allowed at the
previous /etc/containers/ location. This override will be used in
place of the normal config if both are present, and exists to
override distro packaged configs without modifying the standard
config.

Signed-off-by: Matthew Heon <matthew.heon@gmail.com>

Closes: #430
Approved by: rhatdan
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