Issue title: Add an option to specify a configuration file when starting clipper
 Issue body: * Currently, users have to issue many `register_model`, `deploy_model`, and `add_application` commands (among others) via clipper_admin after starting clipper. 

* These commands need to be re-issued after models are stopped, Redis is cleared, etc.

* It would be helpful to allow users to specify a set of models and applications to deploy and register, respectively, through a configuration file. This configuration file could then be specified via the call to `start_clipper`.
 Comments: 
 Comment 0: @dcrankshaw This issue belongs to Nikhil Duggirila. He needs to be added as a collaborator before I can assign this to him.
 Comment 1: I noticed that the PR (#389) for this got abandoned... any chance this is still up for grabs?
 Comment 2: @gmittal Hi! We would want to have more sophisticated design for this feature. We will definitely add this feature sooner or later. I will add this to the feature request and start working on it soon. 
{'source': 'GithubIssues', 'repo_name_with_owner': 'ucbrise/clipper', 'issue_id': 289746463, 'repo_stars': 1399.0}