dashboard: buildlet client, server, and gomote enhancements

- get tarballs out of buildlets
- gomote can pass arguments to buildlet's exec handler
- buildlet's exec handler can run system-level commands
- hard-code GOROOT_BOOTSTRAP to be "go1.4" under the workdir
- adjust MTU size on GCE

etc

Change-Id: I73e18b7a5e395a889f5ee93ba9850d331ffb7812
Reviewed-on: https://go-review.googlesource.com/3052
Reviewed-by: Andrew Gerrand <[email protected]>
9 files changed