Configuration loading of file or plugins need to be able to handle a set of paths

To help with better docker/container setups. It would help a lot if trafficserver can load configuration files and plugins from more than one directory.

Currently plugins can be worked around via defining plugin.cfg to load from absolute paths. However this has to set as configuration files are a all or nothing setup. It would add great value to allow a image to define a set of default configs, while allowing the user to compose only the set of config they need to change in a different volume in which that can mount to a define point that traffic server would also be able to look in. This avoids the needs for user to define a new image in which they have to add there layers, but instead define a volume that is composed in from a default and trusted image