Remove kubebuilder:pruning:PreserveUnknownFields annotation from CassandraDatacenterTemplate

[Miles-Garnsey]

What this PR does:

Removes kubebuilder:pruning:PreserveUnknownFields, which allows CRs to skip validation and can lead to silent errors.

Which issue(s) this PR fixes:
#652

Checklist

• Changes manually tested
• Automated Tests added/updated
• Documentation added/updated
• CHANGELOG.md updated (not required for documentation PRs)
• CLA Signed: DataStax CLA

[Miles-Garnsey]

I made some changes which avoid an NPE here due to the config being nil. I'm still seeing a bunch of errors from the unit tests.

One of the failures is here, where we see that removing the annotation leads to quite a different output in terms of the final CassandraConfig object:

expected: &gabs.Container{object:map[string]interface {}{"cassandra-env-sh":map[string]interface {}{"additional-jvm-opts":[]interface {}{"-Dcassandra.system_distributed_replication=dc1:3,dc2:3", "-Dcom.sun.management.jmxremote.authenticate=true"}}, "cassandra-yaml":map[string]interface {}{"authenticator":"PasswordAuthenticator", "authorizer":"CassandraAuthorizer", "num_tokens":16, "role_manager":"CassandraRoleManager"}}}
actual  : &gabs.Container{object:map[string]interface {}{"cassandra-env-sh":map[string]interface {}{"additional-jvm-opts":[]interface {}{"-Dcassandra.system_distributed_replication=dc1:3,dc2:3", "-Dcom.sun.management.jmxremote.authenticate=true"}}, "cassandra-yaml":map[string]interface {}{"authenticator":"PasswordAuthenticator", "authorizer":"CassandraAuthorizer", "concurrent_reads":4, "concurrent_writes":4, "num_tokens":16, "role_manager":"CassandraRoleManager"}, "jvm-server-options":map[string]interface {}{"max_heap_size":1.073741824e+09}, "jvm11-server-options":map[string]interface {}{"garbage_collector":"G1GC"}}}

Complicating interpretation of the above output is that the actual and expected fields are swapped in the assertion. Once we take account of this, what appears to be happening is that the assertion is expecting far more fields to be defined than are actually being defined - presumably because they are "unknown fields".

[Miles-Garnsey]

Fixed under #710