airship document plugin is intended to be executed as an exec plugin
for kustomize document model.
Environment variable is used to gather plugin configuration. Plugin to
execute is determined based on group-version-kind specified in plugin
configuration. Each airship plugin must implement plugin interface.
Relates-To: #173
Change-Id: I4f6c3b5be140c0d8fd7519f1cedd33de1cef662c