Add more validation for OCICluster

[joekr]

What would you like to be added:
Now that #55 has been merged we need to work on adding more validation checks for things like the following:

• NetworkSpec (valid CIDRs)
• Region is valid (might need to call to OCI for validation or the OCI SDK might have pre-flight validation we could use)
• ClusterName (length?)
• For cluster update region is considered an immutable field. We will want to handle other immutable fields

Why is this needed:
Now that #55 is merged we should expand upon our validation. This issue isn't to 100% cover all possibilities, but rather cover what we can now that make sense.