allow builder logfiles to be separated by instance

If multiple builder instances are running in the same
directory, we don't want them to use the same log file
names.  Changed code to

- respect the --stdoutLog=... command line option if
  it is given

- use a builder2.log.prefix system property to allow an
  optional prefix to be used on the main log4j filename
Merge branch 'master' of git@github.com:daveho/CloudCoder.git