Enable the bosh_micro_enabled parameter

Currently, 'bosh_micro_enabled' is hardcoded to 'yes' in `builder_options.rb`, so the setting in `settings.bash` is useless. This parameter is used to control if the stemcell includes the micro-bosh package or not when building stemcell. It will reduce ~100 mb size if excluding the micro-bosh package from stemcell.

In this modification, we removed the hardcode in `builder_options.rb`, and re-organized the code of `apply.sh`, `config.sh` according to the 'bosh_micro_enabled' setting value. Then, the default value is kept 'yes' and user can control the stemcell to include micro-bosh package or not through the 'bosh_micro_enabled' setting in `settings.bash`.