Add a doc about bootstrapping.

After working through this for the third or fourth time, I thought it was time to write it down.
Not anymore (very recent change). We now:
 - locate tools available externally (matching constraints)
 - determine which tools can be built and uploaded to make up shortfall in above

If the tools are external, then the bootstrap machine will download them. Otherwise, the CLI will copy them to the target machine when it starts it up.

When the bootstrap agent comes up, it copies the tools into environment storage. It'll also clone the tools for each series of the same OS (for the time being at least, while each series' tools are equivalent).