Add requirements and features for scheduling on clouds

[bbarker]

Add requirements inline below:

VM-preferences and requirements

Each cloud provider has different node types, complicating the problem somewhat. Each particular container type or service type (when there is a distinction between the two) should have a poset of node types for at least the cloud provider being used. A configuration provider may support more than one cloud provider. What might be nicer is if we could directly deal with memory, cpu, and coprocessor requirements/preferences, so that this could be specified in a more cloud-agnostic and user-transparent fashion: need to check jclouds API etc

Configuration format

Currently looking at dhall; for the jvm/Eta, will track typelead/eta#704

[bbarker]

Regarding "VM-preferences and requirements", it is unlikely jclouds will be enough, at least for AWS, though OpenStack should be possible, either directly through jclouds or the openstack API; several AWS options were discussed on slack, including scraping https://aws.amazon.com/ec2/instance-types/ or https://ec2instances.info/, or using the pricing API (http://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/price-changes.html).