CLI: set localhost as default for database hostname in verdi setup

The default was actually being defined on the option, however, it was
taken from the `pgsu.DEFAULT_DSN` dictionary, which defines the database
hostname to be `None`. Still, 9 out of 10 times the database is on the
localhost so not having this as a default is kind of annoying and
unnecessary.

aiida/cmdline/params/options/__init__.py

@@ -249,7 +249,7 @@ def decorator(command):
     '--db-host',
     type=types.HostnameType(),
     help='Database server host. Leave empty for "peer" authentication.',
-    default=DEFAULT_DBINFO['host']
+    default='localhost'
 )
 
 DB_PORT = OverridableOption(

aiida/cmdline/params/options/commands/setup.py

@@ -259,7 +259,7 @@ def get_quicksetup_password(ctx, param, value):  # pylint: disable=unused-argume
 
 SETUP_DATABASE_HOSTNAME = QUICKSETUP_DATABASE_HOSTNAME.clone(
     prompt='Database host',
-    contextual_default=functools.partial(get_profile_attribute_default, ('database_hostname', DEFAULT_DBINFO['host'])),
+    contextual_default=functools.partial(get_profile_attribute_default, ('database_hostname', 'localhost')),
     cls=options.interactive.InteractiveOption
 )