Use Gomega matcher in Eventually and check pods' labels

[masa213f]

This PR makes the following changes to the envtest.

1. Use Gomega matcher in Eventually block.
   It's just a refactoring.
2. Check the pods' metadata in Eventually or Consistently blocks.
   The pods' metadata are updated asynchronously with MySQLCluster's .status.
   With the current implementation, pods's metadata are checked just once using Expect.
   Therefore, if there is a delay in updating the pods' metadata, the test may fail.
3. Add more checks for pods's labels.
   Preparation for Kill old connections when demoting primary #561

Signed-off-by: Masayuki Ishii [email protected]

[zoetrope]

LGTM