support k8s network "exec" plugins

[jdef]

High level:

• support the kubelet network plugin config options (pass them from scheduler --> executor)
• ship required plugin binaries from (somewhere) to the executor so that the kubelet can execute the plugin

Goals:

• eliminate hardcoded path: network plugins directory is statically defined, outside the mesos sandbox
• support multiple network plugins
• specify plugins at scheduler startup time (via flags) and pass through to kubelet-executor
• may want to leverage an external artifact server (#202)
• should be able to bundle all plugins into a single archive (.tar.gz), single artifact download
• plugins should be run from the sandbox, not an "external" host directory

Depends on #197

[jdef]

the kubelet network plugin model has changed significantly. this ticket needs re-evaluation