Keep configuration of data stores path

[forman]

Expected behavior

When doing software updates, Cate should take over a compatible data_stores_path configuration setting from an existing installation.

Actual behavior

After any Cate update or installation, the data_stores_path configuration setting is set to its default values $home/.cate/data_stores.

Specifications

0.9.0.dev7

[forman]

Introduced new approach in 2.0.0.dev5: we now always read default ~/.cate/conf.py first, then let an existing ~/.cate/<version>/conf.py overwrite defaults. If not exists, we will write a default ~/.cate/conf.py but not a ~/.cate/<version>/conf.py. However, users may create one.