mapistore/py: Initial implementation for OXIO adapter backend
mapistore_test: Add call to list_context - currently it is a non-failure
mapistore_test: Add context with our test ID, so we could search for it later

With this backend now correctly resolves 0xdeadbeef to opt_uri
in Context.get_path(fmid)
mapistore/py: Print folder ids as HEX when logging
mapistore/py: Don't print error when we don't have a path for given FMID

It is just backend don't know about this FMID, and it is responsibility
for the caller to decide what to do - is it critical or not
mapistore_test: Helper to find any child FMID for a folder

It is to be used for testing later
mapistore/oxio: Fix exception in Folder.create_folder()