Ref T2015. Depends on D10395. This allows custom attributes to be defined against the Amazon EC2 blueprint (such as "this host provides program XYZ"). In the "Lease Host" step, builds can now request hosts with specific custom attributes.

(platform needs to remain a seperate attribute as it defines how commands are invoked)

Tested it with two EC2 blueprints with different attributes. Saw it allocate against the correct EC2 blueprint when attributes were specified. Saw it fail when no blueprint could satisfy the custom attributes.