remove spec.mongod section and move encryptionKeySecret to new variable

As per https://jira.percona.com/browse/PSPEC-9 we have deprecated spec.mongod section.

In 1.12 release we are going to remove it, and only keep:

  mongod:
    security:
      encryptionKeySecret: my-cluster-name-mongodb-encryption-key

We keep it for backward compatibility and introduce new variable instead:

spec:
  secrets:
    encryptionKey:

See ITD 3.1 in spec for more details.

• The key is always generated.
• Encryption is enabled by default, but can be disabled by the user through the configuration section.
• If both spec.secrets.encryptionKey and mongod.security.encryptionKey are specified, then spec.secrets.encryptionKey has more priority and should be used.

We give users time till release 1.14 to migrate to the new option and remove the mongod section completely in 1.14.