Renames *.rst files in /rally/doc/samples/tasks

Renames all .rst files in /rally/doc/samples/tasks
to README.rst.
And moves /rally/doc/samples/support to
/rally/doc/samples/tasks

Closes-bug: #1351599

Change-Id: I675266292bc1fbb62374ce0d12e6d06fbf7a8e3e