Setup the maven proxy inside of the builder

[lburgazzoli]

You can now configure maven proxy in the camel-k platform:

apiVersion: camel.apache.org/v1alpha1
kind: IntegrationPlatform
metadata:
  labels:
    app: camel-k
  name: camel-k
spec:
  build:
    proxies:
    - active: false
      host: my-host
      id: my-proxy
      port: 8080
    - active: false
      host: my-host
      id: my-proxy-2
      port: 8080
      usernameFrom:
        configMapKeyRef:
          key: proxy-user
          name: maven-proxy

Notes:

• there's no cli option at the moment
• proxy can be configured on the platform only, there's no proxy configuration on integration or integration context

Fixes #706

[lburgazzoli]

will rethink it