Add temporary support for loading of new process node ORM classes

[sphuber]

Fixes #2187

The loading of the correct ORM class for a given entry in the DbNode table
is based on the type column, but its interpretation is still in a half-way
state. The plugin loader will first attempt to reverse engineer it into an entry
point and if that works load the correspondingly class. If that fails, it is
interpreted as an internal class and will attempt to load it directly as a module
path.

Adding the new process node ORM classes, will need to be supported by the plugin
loader, so here we add their entry points. We do this in the aiida:node group
but note that in the future the entire system will be changed to solely rely on
entry points, at which the format of the type column can be changed to be a
fully qualified entry point string, just as the process_type column already is.

[coveralls]

Coverage increased (+0.7%) to 67.255% when pulling 2f683d6 on sphuber:fix_2187_temporary_load_support_node_process into 9ad164e on aiidateam:provenance_redesign.

[coveralls]

Coverage increased (+0.7%) to 67.255% when pulling 2f683d6 on sphuber:fix_2187_temporary_load_support_node_process into 9ad164e on aiidateam:provenance_redesign.