Define correct type string for CalcJobNode instances

[sphuber]

The type string of a CalcJobNode should now be just that of the
node class and not the sub class. Since the users are currently still
sub classing the node class and not a process class, we have to make
a special exception when generating the type string for the node.

Conversely, this type string, stored in the type column of the node,
should be used to load the CalcJobNode class when loading from
the database. The only classes that can legally exist in a database, and
therefore be loaded, are defined in aiida-core. Therefore, using the
entry point system to map the type string onto an actual ORM class is no
longer necessary. We rename the aiida.plugins.loader.load_plugin
function to the more correct load_node_class, which given a type string,
will return the corresponding ORM node sub class.

Note that the whole machinery around generating type and query strings
and loading the nodes based on them is still somewhat convoluted and
contains hacks for two reasons:

1. Data is not yet moved within the aiida.orm.node sub module and as
   a result gets the data.Data. type string, which will not match the
   node.Node. type when sub classing in queries.

2. CalcJobProcesses are defined by sub classing JobCalculation
   Until the user directly define a Process sub class that uses the
   CalcJobNode as its node class, exceptions will have to be made.

If these two issues are addressed, a lot of the code around type strings
can be simplified and cleaned up.

[coveralls]

Coverage decreased (-0.005%) to 69.026% when pulling 9baed10 on sphuber:fix_1603_plugin_loader into dd0b1b9 on aiidateam:provenance_redesign.