Extend status_description column in Clusters tables

Convert column type to
 * Text for psql - unlimited size
 * LongText for mysql - max size 2^32

Also fixed template for autogenerated migrations: added a blank line to
avoid pep8 violation.

Closes-Bug: #1338933
Change-Id: Ie4c4f57aeb7ff03aa7f6077629f6463a136f8f4b
Merge "Extend status_description column in Clusters tables"