add an `append_timestamp` configuration value

This value will be used to indicate whether a timestamp should be 
appended to the build version.
wire `append_timestamp` config value into Omnibus::BuildVersion

For backward compatibility we will still prefer the value passed in via
the `OMNIBUS_APPEND_TIMESTAMP` environment variable.
set Omnibus::Config.append_timestamp based on --timestamp

This will preserve the existing behavior of leaving the timestamp off
the generated build version with the `--no-timestamp` CLI option.
print the build version when the build commences
Merge branch 'CHEF-4112'