Handle symlinks on build/projects path

[agis]

Currently, if data_path is a symlink, docker on Mac complains with:

$ ./mistry-cli build --host localhost --project simple
2018/04/12 13:21:47 Error creating job: Error building {project=simple params=map[] group= id=3dc923b}: work: could not start docker container; Error response from daemon: invalid mount config for type "bind": bind source path does not exist
, http code: 500

This is due to docker/for-mac#1298.

We should resolve symlinks after reading the configuration, and update it accordingly before booting the server.