add --force-local option to tar cmd so Windows paths like D:/boshrelease...

This change is to make ```bosh upload release boshrelease-cf-176.tgz``` work on Windows.

Currently:
```
D:\bosh-lite>bosh --no-color upload release boshrelease-cf-176.tgz
Verifying release...
validation.rb: about to perform_validation
release_tarball.rb: allow_sparse: true
File exists and readable                                     OK
Extract tarball                                              FAILED
```
Debugging uncovered that BOSH uses the tar command to extract the archive, resulting in the following arguments on Windows:
```tar -C C:/Users/DiesK/AppData/Local/Temp/d20140808... -xzf D:/bosh-lite/boshrelease-cf-176.tgz```

tar interprets "D:/blah" as a remote file due to the colon. The '--force-local' option makes it interpret as if the "archive file is local even if it has a colon". After this change, this command completes fine on Windows.