Replacing RunnerPodSpec with corev1.PodSpec

[luizbafilho]

fixes #786

To allow full customization of the runner pod, this PR replaces the custom RunnerPodSpec type with corev1.PodSpec, and uses the mergo package to merge the default settings with the user's ones.

[squaremo]

I understand the motivation, and I can see the problem-- PodSpec has required fields, and as soon as you are supplying a PodSpec value you will have to provide all of those. What you need is PodSpec but with everything marked optional, and I can think of some ways of getting that:

1. make a copy of the PodSpec type locally and change all the annotations
2. patch the generated CRD schema so that the fields are all optional
3. use a freeform-JSON type for the field

All of these have obvious problems. Patching the schema might have the least serious problems, since schema changes can be caught during build.

[squaremo]

4. If it's not a requirement to be able to patch anything in the pod spec, then just including the fields known to be required would be fine.

[yiannistri]

Closing in favour of #910