Support finer grained security capabilities

[bodom0015]

Problem

We currently only have 2 levels of security: default and privileged. Since Kubernetes supports very fine-grained security features, we should support those individually.

Fixes #266

Approach

Replace privileged boolean on our Spec object with a securityContext field that can be passed directly through to Kubernetes.

NOTE: Since this is an admin-only field, there is no API validation or UI component to this PR.

How to Test

1. minikube start
2. docker run --name=etcd -p 4001:4001 -d ndslabs/etcd:2.2.5 /usr/local/bin/etcd \ --bind-addr=0.0.0.0:4001 \ --advertise-client-urls=http://127.0.0.1:4001
3. Configure apiserver.json to point to minikube and etcd
4. Checkout, build branch locally, then run: ./build.sh local && ./build/bin/apiserver-darwin-amd64
5. Use Postman to login as the admin, change log level to 4, create a user, verify the email address, and approve the account
6. Use Postman to add the full.json spec to the catalog
7. Use Postman to create a stack from the fulltest spec
8. Use Postman to trigger StartStack on the stack you just created
9. Check the log output of the API server
   • You should see the full template of the stack that is being created
   • You should see securityContext listed under your container (not the pod itself)
10. Verify with kubectl get pods -n <user> -o yaml
    • You should see securityContext listed under your container (not the pod itself)

NOTE: On second thought, I should probably add a new set of Postman tests for the security