add "cargo xtask local" to run buildomat locally

[emilyalbini]

This PR adds the cargo xtask local family of commands to run a local instance of buildomat with minimal configuration.

The centerpiece if the cargo xtask local setup command, backed by the xtask-setup crate (it's split from xtask as it requires a lot of dependencies). It currently supports configuring buildomat-server, buildomat-github-server, and buildomat-factory-aws, and at startup asks which components the user wants to be configured.

There are then the cargo xtask local BINARY commands, which invokes the binary with the flags required to use the local setup. cargo xtask local buildomat also gives access to the CLI, pointed to the local server.

Other changes included in the PR:

• Added a BUILDOMAT_CONFIG environment variable to the buildomat CLI to point to a separate configuration file, so that cargo xtask local buildomat can transparently use the correct credentials.
• Made specifying the SSH key optional in the AWS factory.
• Added the ability to allocate public IPs to instances in the AWS factory.