Implement fixture for create_store

[mih]

In the old code base this is a helper function from datalad.customremotes.ria_utils. It takes a IO abstraction and build a RIA store serverside.

We should not continue using this helper, and instead use a fixture.

This fixture can utilize a local directory to set such store up, if there is one. Otherwise it can use SSH-based commands. There is no point in forcing a test developer to deal with this complexity individually.

This issue becomes apparent in #58

[adswa]

Implemented for

• local: Add fixture create_store_local to set up a store skeleton locally #97
• ssh: New create_store_ssh fixture #95
• http

[mih]

Fixed via #97