Add http/https proxy system environment variables for both staging and i...

...nstance container

Based on the [discussion](https://groups.google.com/a/cloudfoundry.org/forum/#!mydiscussions/vcap-dev/RTfJ_-WfwDI) in Google Groups, add system environment variables support for http/https proxy for staging and instance container.

Example configuration is as follows
```
staging:
  enabled: true
  environment:
    PATH: /usr/local/ruby/bin
    BUILDPACK_CACHE: /var/vcap/packages/buildpack_cache
  memory_limit_mb: 1024
  disk_limit_mb: 2048
  disk_inode_limit: 200000
  cpu_limit_shares: 512
  max_staging_duration: 900 # 15 minutes
  http_proxy: http_proxy_url
  https_proxy: http_proxy_url
  no_proxy: localhost,127.0.0.1

instance:
  disk_inode_limit: 200000
  memory_to_cpu_share_ratio: 8
  max_cpu_share_limit: 256
  min_cpu_share_limit: 1
  http_proxy: http_proxy_url
  https_proxy: https_proxy_url
  no_proxy: localhost,127.0.0.1
```