entity initializers to define SSH sensors and effectors from yaml

migration of https://github.com/brooklyncentral/brooklyn/pull/1446

to review, start by looking at the documentation and the example therein, then look at the commits individually to see how it is built up
Is this a comments about what the method does (because effector does `child.invoke(DOUBLE_CHECK...)`), or is it a TODO?
If a comment, then change also "also asserts it works...`, and point folk at the line of `child.invoke`. If a todo, then prepend comment with `TODO `.