Add support for "peer" authentication with PostgreSQL

PostgreSQL allows "peer" authentication to connect to the database. This
is signaled by a `hostname` that is set to `None` through `pgsu`. In this
case, the `hostname` part of the connection string of the SQLAlchemy
engine should be left empty. If it is `None` it is converted to an empty
string otherwise it would be converted to the string literal "None".

aiida/backends/utils.py

@@ -24,12 +24,16 @@ def create_sqlalchemy_engine(profile, **kwargs):
     from sqlalchemy import create_engine
     from aiida.common import json
 
+    # The hostname may be `None`, which is a valid value in the case of peer authentication for example. In this case
+    # it should be converted to an empty string, because otherwise the `None` will be converted to string literal "None"
+    hostname = profile.database_hostname or ''
     separator = ':' if profile.database_port else ''
+
     engine_url = 'postgresql://{user}:{password}@{hostname}{separator}{port}/{name}'.format(
         separator=separator,
         user=profile.database_username,
         password=profile.database_password,
-        hostname=profile.database_hostname,
+        hostname=hostname,
         port=profile.database_port,
         name=profile.database_name
     )