Add build command

Add a `build` command to the `elevage` tool.

Given a fully described platform, infrastructure and environment, execute the necessary commands to provision the specified virtual machines, running jobs concurrently until all jobs have completed.

    $ elevage help build
    Usage:
      elevage build TARGET

    Provision vms in an environment definition

    $ elevage build --help
    Usage:
      elevage elevage:build ENV

    Options:
      -a, [--all], [--no-all]                      # Build everything for the named environment
      -t, [--tier=TIER]                            # Build everything for the specified tier in the named environment
      -c, [--component=COMPONENT]                  # Build all nodes for the specified component
      -n, [--node=N]                               # Build the single specified node
      -C, [--concurrency=N]                        # Maximum number of simultaneous provisioning tasks
                                                   # Default: 8
      -l, [--logfiles=LOGFILES]                    # Path where log files should be written
                                                   # Default: logs
      -v, [--verbose], [--no-verbose]
          [--dry-run], [--no-dry-run]              # Do not actually do anything, just display the commands that would be run.
          [--vsuser=VSUSER]                        # Username for vSphere
                                                   # Default: svc_provisioner
          [--vspass=VSPASS]                        # Password for vSphere
                                                   # Default: changeme
      -x, [--ssh-user=SSH-USER]                    # Unix username for ssh for chef-client bootstrap (must have sudo NOPASSWD access for root)
                                                   # Default: knife
      -i, [--ssh-key=SSH-KEY]                      # Path to SSH private key for ssh username for key-based authentication
                                                   # Default: knife_rsa
      -t, [--template-file=TEMPLATE-FILE]          # File to be used as the chef-client bootstrap template script
                                                   # Default: chef-full.erb
      -b, [--bootstrap-version=BOOTSTRAP-VERSION]  # Version of chef-client to bootstrap on node
                                                   # Default: 11.4.0