roomservice: Add lightweight dependencies to repositories

Roomservice can already fetch your cm_<device> without the need for a
manifest entry.

However, when working with common repositories, there is no way of
actually fetching them without adding to the manifest. This patch
introduces a lightweight dependency system. Each repository can have a
cm.dependencies in the following json format:

[
  {
    "repository": "repository_name_on_cm_organization"
    "target_path": "target/path"
  },
  ...
]

For instance, for cm_anzu I need android_device_semc_msm7x30-common and
android_device_semc_mogami-common. I would add both to cm.dependencies
as follows:

[
  {
    "repository": "android_device_semc_msm7x30-common",
    "target_path": "device/semc/msm7x30-common"
  },
  {
    "repository": "android_device_semc_mogami-common",
    "target_path": "device/semc/mogami-common"
  }
]

Roomservice would then fetch the anzu repository, parse the dependency
files and add/fetch/sync these additional repositories if they don't
exist already.

This also adds pretty printing to the output xml.

Change-Id: I9cc847adfc717a06439bc6094213ed6492343158