Adds a new mechanism to load providers from any gem or file path. (#263)

* Adds ability to load only providers used in config file

PROVIDER_API.md

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+# Provider API
+
+## Create a new provider gem from scratch
+
+### Requirements
+1. the provider code will need to be in lib/vmpooler/providers directory of your gem regardless of your gem name
+2. the main provider code file should be named the same at the name of the provider. ie. (vpshere == lib/vmpooler/providers/vsphere.rb)
+3. The gem must be installed on the same machine as vmpooler
+4. The provider name must be referenced in the vmpooler config file in order for it to be loaded.
+5. Your gem name or repository name should contain vmpooler-<name>-provider so the community can easily search provider plugins
+   for vmpooler.
+### 1. Use bundler to create the provider scaffolding
+
+```
+bundler gem --test=rspec --no-exe --no-ext vmpooler-spoof-provider
+cd vmpooler-providers-spoof/
+mkdir -p ./lib/vmpooler/providers
+cd ./lib/vmpooler/providers
+touch spoof.rb
+
+```
+
+There may be some boilerplate files there were generated, just delete those.
+
+### 2. Create the main provider file
+Ensure the main provider file uses the following code.
+
+
+```ruby
+# lib/vmpooler/providers/spoof.rb
+require 'yaml'
+require 'vmpooler/providers/base'
+
+module Vmpooler
+  class PoolManager
+    class Provider
+      class Spoof < Vmpooler::PoolManager::Provider::Base
+      
+      # at this time it is not documented which methods should be implemented
+      # have a look at the vmpooler/providers/vpshere provider for examples
+
+      end
+
+    end
+  end
+end
+  
+
+```
+
+### 3. Fill out your gemspec
+Ensure you fill out your gemspec file to your specifications.  If you need a dependency please make sure you require them.
+
+`spec.add_dependency "vmware", "~> 1.15"`.
+
+At a minimum you may want to add the vmpooler gem as a dev dependency so you can use it during testing.
+
+`spec.add_dev_dependency "vmpooler", "~> 1.15"`
+
+or in your Gemfile
+
+```ruby
+
+gem 'vmpooler', github: 'puppetlabs/vmpooler'
+```
+
+Also make sure this dependency can be loaded by jruby.  If the dependency cannot be used by jruby don't use it.
+
+### 4. Create some tests
+Your provider code should be tested before releasing.  Copy and refactor some tests from the vmpooler gem under 
+`spec/unit/providers/dummy_spec.rb`
+
+### 5. Publish
+Think your provider gem is good enough for others?  Publish it and tell us on Slack or update this doc with a link to your gem.
+
+
+## Available Third Party Providers
+Be the first to update this list.  Create a provider today!
+
+
+## Example provider
+You can use the following [repo as an example](https://github.com/logicminds/vmpooler-vsphere-provider) of how to setup your provider gem.
+