Implement configurable SSH device

Setup labgrid_config_toolchain, platform and constraints for a new configurable device, which would allow a user provide the NetworkService resource options of the config.yaml by passing it in to the toolchain.

This new device is to be used with any device that is capable of communicating via SSH.

It should make use of bazel string flags to take in the parameters from the Bazel CLI and pass them through to the labgrid_config_toolchain.

A toolchain_test/build_test should be used to confirm that passing in options into the labgrid_config_toolchain creates a config.yaml that is expected.

In terms of implementation we will need a new labgrid_config_toolchain under labgrid/toolchain/config/configurable which will need some way to pass in the string_flag into the config.yaml.

The config.yaml for the qemu toolchains make use of the labgrid !template syntax, which requires setting values as environment variables, which would be one approach.

A new device constraint under labgrid/constraint/BUILD.bazel called 'configurable', and a new platform in labgrid/platform/BUILD.bazel possibly also called 'configurable`.