Fuel menu modifications for dynamically build bootstrap

Fuel menu should be extended with "Skip" checkbox
to allow user skip bootstrap build process. This option
is helpful to save finish installation faster if user is sure
that default bootstrap image can't be used on his environment
and custom bootstrap image is required.
Fuel menu should be extended with "Skip" checkbox
to allow user skip bootstrap build process.
This option is helpful to save finish installation faster if user is sure
that default bootstrap image can't be used on his environment
and custom bootstrap image is required.
SKIP_BS_BUILD variable should be passed to env.

Also, we need to be able to pass the configuration from fuel-menu to /etc/fuel/astute.yaml and env.
First-required list of extra variable, which should be avaible for passing from menu:

BOOTSTRAP:
  HTTP_PROXY: ''"
  HTTPS_PROXY: ''
  flavor: ubuntu
  repos:
    -
      name: "ubuntu"
      section: "main universe multiverse"
      uri: "http://archive.ubuntu.com/ubuntu/"
      priority: null
      suite: "trusty"
      type: "deb"
   . . .

Acceptance criteria:
* It should be possible to configure https proxy
* It should be possible to ovveride default repos and suits
* It should be possible to configure skipping build bootstrap image

Verification:
* Manual testing