OS-15679: Fix cells mapping project property
To avoid bug described in https://its.cern.ch/jira/browse/OS-15679 where cells_mapping was stored as follows:
+------------+--------------------------------------+
| properties | {'cells_mapping': 'gva_project_053'} |
+------------+--------------------------------------+
Instead of:
+---------------+-----------------+
| cells_mapping | gva_project_053 |
+---------------+-----------------+