Delete database test helpers

[gibizer]

This is necessary to remove a dependency cycle. The helpers are moved to mariadb-operator/api module.

PR merging order:

• Copy the helper code to mariadb-operator
  - Move database test helpers from lib-common to /api mariadb-operator#155
• Adapt all the service operators to use the helpers from mariadb-operator instead of lib-common. Can be merged in parallel:
  • [envtest]Use database helpers from mariadb-operator nova-operator#548
  • [envtest]Use database helpers from mariadb-operator keystone-operator#316
  • [envtest]Use database helpers from mariadb-operator placement-operator#82
  • [envtest]Use database helpers from mariadb-operator glance-operator#324
  • [envtest]Use database helpers from mariadb-operator neutron-operator#215
  • [envtest]Use database helpers from mariadb-operator manila-operator#148
  • [envtest]Use database helpers from mariadb-operator ironic-operator#339
  • [envtest]Use database helpers from mariadb-operator heat-operator#248
• Merge this PR to delete the unused helper from lib-common and therefore remove the mariadb dependency
• Update the envtest.md in the docs repo
  - [envtest]note that test helpers are moved dev-docs#64

[gibizer]

/hold we need to adapt the service operators first

[stuggi]

/lgtm