Pin SCION Version and remove hard dependency on scionlab packages

[martenwallewein]

Idea:

• Provide a method on the SCION layer and per node to configure how to install SCION
• The one on the SCION layer should set the default for all nodes, on the node level its possible to overwrite this
• The method should accept a config object that specifies how to install SCION, something like this (just an example)

# Global configuration
scion.SetBuildConfiguration({
   mode: "build" | "release", # Either build from source or use release version
   releaseLocation: "./bin/" | "https://github.com/scionproto/scion/releases/download/v0.12.0/scion_0.12.0_amd64_linux.tar.gz", # Can be local folder or URL to zip archive
   gitRepoUrl: "https://github.com/scionproto/scion", # For mode == build
   checkout: "v0.12.0", # Tag, commit or branch
   version: "v0.12.0" # Explict version information in case this can not be 
})

For mode == release the binaries should be either copied into the container from the local folder or extracted in case there was a link configured to an archive. For mode == build there should be a way to build SCION from the configured commit, branch or tag in checkout without rebuilding it for every node.

A similar method should be available to override the global config per node.

Default config should be:

# Global configuration
scion.SetBuildConfiguration({
   mode: "release", 
   releaseLocation: "https://github.com/scionproto/scion/releases/download/v0.12.0/scion_0.12.0_amd64_linux.tar.gz",
   version: "v0.12.0"
})