Change directory layout for dynamips.

This create a directory by node to allow feature like save as
to work when running on multiple compute. Benefits now dynamips work
like other nodes. Old topologies are migrated.

Ref https://github.com/GNS3/gns3-gui/issues/1761

docs/file_format.rst

@@ -23,7 +23,8 @@ A minimal version:
 
 The revision is the version of file format:
 
-* 6: GNS3 2.0
+* 7: GNS3 2.0
+* 6: GNS3 2.0 < beta 3
 * 5: GNS3 2.0 < alpha 4
 * 4: GNS3 1.5
 * 3: GNS3 1.4