Hook spec validation only runs for hooks in clusters, not hooks in instancegroups

[geekofalltrades]

------------- BUG REPORT TEMPLATE --------------------

1. What kops version are you running? The command kops version, will display
   this information.

   $ kops version
   Version 1.8.0 (git-5099bc5)
   

2. What Kubernetes version are you running? kubectl version will print the
   version if a cluster is running or provide the Kubernetes version specified as
   a kops flag.

   N/A

3. What cloud provider are you using?

   AWS

4. What commands did you run? What is the simplest way to reproduce this issue?

   Add the following broken hook spec to both a cluster (e.g. with kops edit cluster) and an instancegroup (e.g. with kops edit instancegroup):

   hooks:
   - name: a-broken-hook
   

5. What happened after the commands executed?

   When attempting to add this broken hook spec to a cluster, you get a validation failure:

   # error populating cluster spec: spec.hooks[0]: Required value: you must set either manifest or execContainer for a hook
   

   However, when adding it to an instancegroup, there is no validation failure.

6. What did you expect to happen?

   Validation should fail when adding this hook spec to an instancegroup, as well as to a cluster.

7. Please provide your cluster manifest. Execute
   kops get --name my.example.com -oyaml to display your cluster manifest.
   You may want to remove your cluster name and other sensitive information.

   N/A

8. Please run the commands with most verbose logging by adding the -v 10 flag.
   Paste the logs into this report, or in a gist and provide the gist link here.

   N/A

9. Anything else do we need to know?

   The problem appears to be that the validateHookSpec function which generates this error only runs during cluster validation, and not during instancegroup validation.

[gambol99]

/assign @gambol99

[gambol99]

Much appreciated @geekofalltrades ... I'll take a look :-) unless you already / doing have a PR?

[geekofalltrades]

I don't have a PR for this, no.