Fix migration script for v1.2 upgrade (#651)

### Feature or Bugfix
- Bugfix

### Detail
- migration script for upgrade to V1.2 had a mistake and is affecting
one customer. Basically the `devStrategy` and `devStages` values were
not backfilled which causes nulls in the RDS table that are not allowed
as this column should contain only non-null values.

In this PR we modify that script for customers that have not updated
yet. It is not 100% clear to me whether we should merge it, but I wanted
to raise awareness of this issue here.

### Relates
- #637 

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license.

backend/migrations/versions/b1cdc0dc987a_fix_template_column_in_table.py

@@ -7,21 +7,61 @@
 """
 from alembic import op
 import sqlalchemy as sa
+from sqlalchemy import orm, Column, String
 from sqlalchemy.dialects import postgresql
+from sqlalchemy.ext.declarative import declarative_base
 
 # revision identifiers, used by Alembic.
 revision = 'b1cdc0dc987a'
 down_revision = '4392a0c9747f'
 branch_labels = None
 depends_on = None
 
+Base = declarative_base()
+
+
+class DataPipeline(Base):
+    __tablename__ = 'datapipeline'
+    DataPipelineUri = Column(
+        String, nullable=False, primary_key=True
+    )
+    devStrategy = Column(String, nullable=True)
+    devStages = Column(postgresql.ARRAY(String), nullable=True)
+
 
 def upgrade():
     # ### commands auto generated by Alembic - please adjust! ###
+    # Modify column types
+    print("Upgrade devStages and devStrategy column types. Updating nullable to True...")
     op.add_column(
         'datapipeline',
         sa.Column('template', sa.String(), nullable=True)
     )
+    op.alter_column(
+        'datapipeline',
+        'devStages',
+        existing_type=postgresql.ARRAY(sa.VARCHAR()),
+        nullable=True
+    )
+    op.alter_column(
+        'datapipeline',
+        'devStrategy',
+        existing_type=sa.VARCHAR(),
+        nullable=True
+    )
+    print("Backfilling values for devStages and devStrategy...")
+    # Backfill values
+    bind = op.get_bind()
+    session = orm.Session(bind=bind)
+    session.query(DataPipeline).filter(DataPipeline.devStrategy is None).update(
+        {DataPipeline.devStrategy: 'gitflowBlueprint'}, synchronize_session=False)
+
+    session.query(DataPipeline).filter(DataPipeline.devStages is None).update(
+        {DataPipeline.devStages: ['dev', 'test', 'prod']}, synchronize_session=False)
+    session.commit()
+
+    print("Backfilling values for devStages and devStrategy is done. Updating nullable to False...")
+    # Force nullable = False
     op.alter_column(
         'datapipeline',
         'devStages',