Add ConfigKey.isReconfigurable

- and adds a builder to BasicConfigKey, because number of options is
  getting too many to just overload the constructor.
Adds ConfigKeys to Policy

- Adds policy.getPolicyType()
- Adds policy.getConfigKey/setConfigKey()
- Adds Configurable trait for policy and entity to implement
BasicConfigKey: support type with generics
Add policy config to REST api

- And renames ConfigResource to EntityConfigResource, and adds
  abstract super type also extended by PolicyConfigResource.
AutoScalerPolicy to use ConfigKeys
Merge pull request #513 from aledsage/feature/policy-configuring

Configuring policies using ConfigKey