Fix #529 2 - "This Session's transaction has been rolled back" (#531)

* Created migration to fix the bug

* Working also on MySQL

* Added support for Vertica Grains (#515)

* Fix #529 1 "This Session's transaction has been rolled back" (#530)

* Fixing the specific issue

* Added an additional fix for a similar error in #529

Background:
- When an object is modified by SQLAlchemy, it is invalidated so need to be fetched again from the DB
- If there's an exception during a transaction, SQLAlchemy performs a rollback and mark the connection as dirty.

Bug:
- When handling exceptions, the exception handler tries to access the name of the cluster in the main object. Since the name has been invalidated due to a write, SQLAlchemy tries to fetch it on a 'dirty' connection and spits out an error. Solution:
- Fetch the information for handling the exception before starting the process.

* Modified the migration function to to automatically detect the the foreign keys based on the signature.
It supports also sqlite using batch migrations

* i18n: Fix typo in Druid cluster broker port label (#512)

* Update models.py (#541)

removing duplicated `user_id` def

caravel/migrations/versions/1226819ee0e3_fix_wrong_constraint_on_table_columns.py

@@ -0,0 +1,50 @@
+"""Fix wrong constraint on table columns
+
+Revision ID: 1226819ee0e3
+Revises: 956a063c52b3
+Create Date: 2016-05-27 15:03:32.980343
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = '1226819ee0e3'
+down_revision = '956a063c52b3'
+
+from alembic import op
+import sqlalchemy as sa
+
+naming_convention = {
+    "fk":
+    "fk_%(table_name)s_%(column_0_name)s_%(referred_table_name)s",
+}
+
+def find_constraint_name(upgrade = True):
+    __table = 'columns'
+    __cols = {'column_name'} if upgrade else {'datasource_name'}
+    __referenced = 'datasources'
+    __ref_cols = {'datasource_name'} if upgrade else {'column_name'}
+
+    engine = op.get_bind().engine
+    m = sa.MetaData({})
+    t=sa.Table(__table,m, autoload=True, autoload_with=engine)
+
+    for fk in t.foreign_key_constraints:
+        if fk.referred_table.name == __referenced and \
+            set(fk.column_keys) == __cols:
+            return fk.name
+    return None
+
+def upgrade():
+    constraint = find_constraint_name() or 'fk_columns_column_name_datasources'
+    with op.batch_alter_table("columns", 
+        naming_convention=naming_convention) as batch_op:
+        batch_op.drop_constraint(constraint, type_="foreignkey")
+        batch_op.create_foreign_key('fk_columns_datasource_name_datasources', 'datasources', ['datasource_name'], ['datasource_name'])
+
+def downgrade():
+    constraint = find_constraint_name(False) or 'fk_columns_datasource_name_datasources'
+    with op.batch_alter_table("columns", 
+        naming_convention=naming_convention) as batch_op:
+        batch_op.drop_constraint(constraint, type_="foreignkey")
+        batch_op.create_foreign_key('fk_columns_column_name_datasources', 'datasources', ['column_name'], ['datasource_name'])
+