Allow charm users to set admission-control

[Bui Hong Ha]

Currently, kube-api's admission-control is fixed at deployment with preset values [1].
This prevents users from passing their own admissions (for example: PodSecurityPolicy).

Also, in v1.15 admission controls, which are set in the charm, are already enabled by default.
Admission control therefore should be passed as additional plugins using flag: --enable-admission-lugins.

# Reference
1. https://github.com/charmed-kubernetes/charm-kubernetes-master/blob/master/reactive/kubernetes_master.py
2. https://kubernetes.io/docs/reference/command-line-tools-reference/kube-apiserver/

[Tim Van Steenburgh (tvansteenburgh)]

https://github.com/charmed-kubernetes/charm-kubernetes-master/pull/55