[issue 161] - Use the :centos7 tag for PostgreSql and MySql images

[fabiobrz]

Description

Fix #161

Type of change

• Bug fix (non-breaking change which fixes an issue)
• Feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change
• Chore (non-breaking change which doesn't affect codebase;
  test, version modification, documentation, etc.)

Checklist

• Code contributed by me aligns with current project license: Apache 2.0
• Pull Request contains a description of the changes
• Pull Request does not include fixes for multiple issues/topics
• Code is self-descriptive and/or documented
• N/A - I have implemented unit tests to cover my changes
• I tested my code in OpenShift

[fabiobrz]

Merging this one without the usual review process, since it is just about a configuration property value hotfix, well documented and that passed all the CI checks.