Fix the default value of variables in the bost_agent stage

As the directory of the stemcell builder was moved, the relative path to the bosh agent was aslo changed. I fixed the relative path according to the current locations.

I also added the default value of `bosh_release_src_dir` and `mcf_enabled`. These values are usually given by the rake task, but when execute the `build_from_spec` script according to the README, these variables cause errors.