sql-connection: make _setup_tables staticmethod

This change enables external usage of the setup_tables method.

Change-Id: I0b4c43f24afd0d7f05e9b16565ec63b650e56eb0
diff --git a/zuul/driver/sql/sqlconnection.py b/zuul/driver/sql/sqlconnection.py
index 4b1b1a2..e478d33 100644
--- a/zuul/driver/sql/sqlconnection.py
+++ b/zuul/driver/sql/sqlconnection.py
@@ -43,6 +43,8 @@
             self.engine = sa.create_engine(self.dburi)
             self._migrate()
             self._setup_tables()
+            self.zuul_buildset_table, self.zuul_build_table \
+                = self._setup_tables()
             self.tables_established = True
         except sa.exc.NoSuchModuleError:
             self.log.exception(
@@ -68,10 +70,11 @@
 
             alembic.command.upgrade(config, 'head')
 
-    def _setup_tables(self):
+    @staticmethod
+    def _setup_tables():
         metadata = sa.MetaData()
 
-        self.zuul_buildset_table = sa.Table(
+        zuul_buildset_table = sa.Table(
             BUILDSET_TABLE, metadata,
             sa.Column('id', sa.Integer, primary_key=True),
             sa.Column('zuul_ref', sa.String(255)),
@@ -84,7 +87,7 @@
             sa.Column('message', sa.TEXT()),
         )
 
-        self.zuul_build_table = sa.Table(
+        zuul_build_table = sa.Table(
             BUILD_TABLE, metadata,
             sa.Column('id', sa.Integer, primary_key=True),
             sa.Column('buildset_id', sa.Integer,
@@ -99,6 +102,8 @@
             sa.Column('node_name', sa.String(255)),
         )
 
+        return zuul_buildset_table, zuul_build_table
+
 
 def getSchema():
     sql_connection = v.Any(str, v.Schema(dict))