Support parallel build (scons -jNN)
- Make sure Actions/Builders can run from tree root.
- Avoid use of chdir=1 in scons actions, instead use 'cd dir && ' in
  all shell commands.
- Use scons and python utilities for path manipulations.
- Add pretty-print for the Sandesh* Builder's
- Fix a couple of things: HOSTNAME; determination of git version;
  dependency on build/bin/sandesh compiler

Change-Id: Ifb531a7cb0777cf74eb4fa368a481c3d1963172e