Enable URL based installation

User can install the xenserver-core packages from an URL, by defining two environment variables:

```
PKG_REPO_LOCATION
SRC_REPO_LOCATION
```

An example call might be:

`PKG_REPO_LOCATION=http://downloadns.citrix.com.edgesuite.net/8453/snapshots/71bcf98ad31851059304c13b381446e01bb100b3/ubuntu/deb/ bash scripts/deb/install.sh`

fixes #315