Canary new annotation

[minherz]

Replace existing multiple canary annotations with a single one: canary-by-predicate. The annotation's value is a simple YAML expression that defines a header, a cookie and a weight to be used for routing to an alternative backend like this:

header:
  name: "<http header name>"
  values:
    - "first possible header value"
    - "second possible header value>"
cookie:
  name:"<cookie name>"
  values:
    - "first possible cookie value"
    - "second possible value>"
weight: <integer value in [0..100]>

The following logic is implemented after parsing and publishing traffic policy:
given all (header, cookie and weight) are defined and header and cookie have multiple values
if header is sent in http request and its value matches ANY of the defined header values
or cookie is sent in http request and its value matches ANY of the defined cookie values
or weight functions <= defined weight
then
route request to alternative backend
end

[minherz]

the work is postponed. the way definitions are delivered isn't good for use with annotations.