agent.ini should be supported by default

Currently, agent.ini file is explicitly provided by the users when creating application instance. The agent.ini file should be created per application instance as its possible for slider-agent instances for specific application instance to have unique configurations.

For example, a user may want to debug agents at the level DEBUG for a specific application instances while not requiring other applications to also run with DEBUG level for agent logs.

Proposal:

• Slider will have a default agent.ini that gets automatically uploaded
• Have folks provide overrides when creating an app
• AM patches the default using overrides and then drops the .ini somewhere in the app instance specific directory (default could be in a well-known place)
• The patched .ini is then handed off to all agent instances through YARN localized file support