Break out orchestration code into standardised interface

[seanmtracey]

For the initial version of Andaime, all of the code (not including bundled shell scripts) is in one file: andaime.go.

This made sense for the proof of concept, as we were only targeting one cloud platform at the time (AWS), but as we expand to support other cloud providers (Azure, GCP, etc.), it would make sense to have platform specific code in its own file/module that can be called from andaime.go with something like a "standard" set of configuration options, and vendor-specific ones.