Standardized admin/consumer configuration file loading/validation.

Basically makes the admin.conf and consumer.conf work like the server.conf.  The consumer.config.py already existed and was being used by the agent.  In this PR, I just expanded it to provide default values and updated the schema for missing values.  Created the admin/config.py following the same pattern.  Last updated the launcher.py and main() methods for both CLIs to use the standard config loading function.

There is no test covering the pulp.client.launcher and too much work to add it now.
Could it check for an environment variable PULP_CONF_DIR to optionally add another directory from the env that would be a substitute for /etc/pulp/?