Revamping template- and task-path

The scheme for finding templates within a task should be:

tasks/<name>.task/<filename>

The task itself must be named:

tasks/<name>.task/metadata.yaml

The change is fourfold:

1. Remove os_version from path.
2. Add .task to folder name.
3. Allow <name> to contain "/" to indicate subfolders.
   • This is mostly relevant for derived tasks referencing a base task.
4. Require task filename be "metadata.yaml".

The problem that sparked this (RAZOR-117) was due to inheritance because parent tasks are specified by "name", not "name" + "os_version". Thus, the required os_version subdirectory is unnecessary. The "name" should be sufficiently qualified instead.