add limit to database relations

[amc94]

Applicable spec:

Overview

Sets a limit to the database relation
#336

Rationale

It was causing charm qa runs to fail

Juju Events Changes

Module Changes

Library Changes

Checklist

• The charm style guide was applied
• The contributing guide was applied
• The changes are compliant with ISD054 - Managing Charm Complexity
• The documentation is updated.
• The PR is tagged with appropriate label (urgent, trivial, complex)
• The docs/changelog.md is updated with user-relevant changes in the format of keep a changelog v1.1.0

solves #336