Properly handle possible `/` at the end of the disk path in `cluster.yaml`

[ikopylov]

trafficstars

Right now regular string concatenation is used to build the path to the alien directory (https://github.com/qoollo/bob/blob/master/bob-backend/src/pearl/settings.rs#L32). This result in double / symbols. Path::join should be used instead.

The same thing happens here: https://github.com/qoollo/bob/blob/master/bob-backend/src/pearl/settings.rs#L267

Probably, there are more places containing similar mistake. All places should be found and fixed