Add tests to IPAM plugin

[KapilYadava]

Fixes #92 for IPAM

[damyan]

Improvements needed:

• squash first 4 commits to a single one, adding some meaningful name (adding some meaningful names for commits/branches/PRs/descriptions is generally a good idea, because those will appear on default branch history)
• enhance the create IPAM IP tests to:
  • create a new IPAM IP for some MAC address
  • update already created IPAM IP for the same MAC, because IP address differs
  • update already created IPAM IP for the same MAC, because subnet differs
• remove redundant creation of default config. idea:
  • change loadConfig() to setup6() in suite_test, so the client is parametrized with the proper configuration. use that config in all straightforward tests
  • delete config creation from plugin_test
  • whenever a different/wrong/malformed/non-existent config is needed: create it in the test and call loadConfig()

Let's discuss the changes in our next meeting