[dist] Fix/Enhance obs_mirror_project

Add back obs_mirror_project script deleted in commit a91447c5af2869ed518f8934357e173bfd723005
and fix/enhance with the following:
1. Include 'optparse' to parse commandline options
2. Verify user inputs from commandline options
3. Handle existing file by comparing file size
4. Download retry up to 3 times in case temporary network interruption
[backend] Move 4 bs_worker config options into BSConfig.pm

1. buildlog_maxsize
2. buildlog_maxidle
3. xenstore_maxsize
4. gettimeout