add ability to generate toolchain config w/o docker

[amkartashov]

Main motivation: generate toolchain configuration for MacOS remote execution (which cannot use docker).

Also can be used on Linux, f.e. when remote workers are VMs and not containers.

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Changes:

• add runner package with Runner interface and 2 implementations: docker runner (almost unchanged code from master) and host runner
• add option --runner
• move some code to options package to avoid circular deps
• add platform options for MacOS
• when output is a directory, remove WORKSPACE from cc subdir

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To make sure that host runner works identically with docker runner, I verified that host runner produces identical toolchain configuration for linux.

In other words, command:

./rbe_configs_gen_linux --runner=host --output_tarball=rbe_default_linux_host.tar --exec_os=linux --target_os=linux --cleanup=false --toolchain_container=l.gcr.io/google/rbe-ubuntu16-04@sha256:f6568d8168b14aafd1b707019927a63c2d37113a03bcee188218f99bd0327ea1

which runs inside container, produces archive with the same md5sum as command:

./rbe_configs_gen --toolchain_container=l.gcr.io/google/rbe-ubuntu16-04@sha256:f6568d8168b14aafd1b707019927a63c2d37113a03bcee188218f99bd0327ea1 --output_tarball=rbe_default_linux_docker.tar --exec_os=linux --target_os=linux

does (here rbe_configs_gen binary is built from current master and runs on the real machine, not container)

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[amkartashov]

@googlebot I signed it!