Passing `--timestamp` without an optional argument it does not append a timestamp

[vkarak]

Whereas, according to the documentation, it should default to %FT%T. This is very easy to reproduce:

./bin/reframe --timestamp -l

  stage directory:   '/Users/karakasv/Repositories/reframe/stage'
  output directory:  '/Users/karakasv/Repositories/reframe/output'

./bin/reframe --timestamp=%T -l

  stage directory:   '/Users/karakasv/Repositories/reframe/stage/15:50:49'
  output directory:  '/Users/karakasv/Repositories/reframe/output/15:50:49'