Ensure cachedproperty descriptor picks up docstrings

Closes-Bug: #1325677

Change-Id: I6366dea5458adec30e945a9834e15da946813461
Rename inject_task_args to inject_atom_args

Since storage injection can target atoms and not
just tasks (just one type of atom) the method naming
is more appropriate when named as atom injection (and
associated variable name changes reflect this as well).

Also add more stringent checking around the names of
atoms that are targeted for injection (ensuring that
the name provided actually exists in the storage unit).

Change-Id: I0ad0178240613fda166ea8fcdc441b37290445f8
Merge "Rename inject_task_args to inject_atom_args"
Merge "Ensure cachedproperty descriptor picks up docstrings"