tapper init: provide a subdir for producers

...to make a decision easier to the user.
tapper init: testplans/ subdir now fine-grained

The ~/.tapper/testplans/ subdir is expected to be used
by the user. To make it easier to provide updates we
now initialize our bundled testplans file-by-file.

Now the user doesn't need to delete the whole directory
to get our bundled new testplan files.
testplans/*/local: allow specifying precondition producers
testplans/*/example03-builder: frontend to testplans/*/local

...to demonstrate arguments to a producer.

An example precondition producer which builds a
file and provides preconditions pointing to the
built file.
example_producer_builder.sh: example for builder

An example precondition producer which builds a
file and provides preconditions pointing to the
built file.
example_kernel_install.sh: example installer

...used inside the produced YAML produced from the
producer 'example_producer_builder.sh'.
changelog++
v4.1.8

        - helloworld: example_kernel_install.sh: example installer
        - helloworld: example_producer_builder.sh: example for builder
        - helloworld: testplans/*/example03-builder: frontend to testplans/*/local
        - helloworld: testplans/*/local: allow specifying precondition producers
        - tapper init: testplans/ subdir now fine-grained
        - tapper init: provide a subdir for producers
        - add notifications for new testrun if requested
        - [TEST] adding notification with testrun
        - [FIX] API docs: new notifications can not be added as a string