Restore behavior of JobCalculation.get_option returning default value

[sphuber]

Fixes #2012

Recently, the various methods to get options from a JobCalculation,
the historically called get_* methods, would return a default value
if the value was not explicitly set as an attribute. These methods were
recently replaced by the generic get_option method and the
JobCalculation.options dictionary, however, by default the get_option
would only return a value if explicitly set. One would have to set the
argument only_actually_set to False to get the default value specified
in JobCalculation.options, if it is defined and not None.

The business logic however, was always calling get_option with default
argument while expecting the default to be returned when not explicitly
set. In addition, certain options whose get methods used to return a
default no longer had a default defined, e.g. append_text.

Here we properly define defaults for the options that always had one as
defined by the old explicit get methods. In addition, the default for
the only_actually_set for get_option is changed to False. The
internal code in general always expects to get defaults even when not
set. Note that this required the code for get_builder_restart to be
adapted as that expects only those options to be set that were actually
defined by the user when constructing the JobCalculation.

[coveralls]

Coverage increased (+1.2%) to 67.81% when pulling 24434ea on sphuber:fix_2012_job_calculation_get_option_default into 36d6aef on aiidateam:develop.