Add Volume permissions initContainer Toggle

[jwc-kta]

While attempting to use this Chart on openshift, I discovered that OpenShift's built-in Security Context Contraints make the SecurityContext setting for this chart unnecessary and when using with persistent storage there is no way to disable the init container responsible for configuring the /storage mount's permissions. With a default security context, no processes can run as root and execute chmod's or chown's to change the permissions of /storage.

As such, on openshift (and maybe other k8s distros), the permissions init container isn't necessary. This PR adds a boolean toggle for disabling this in the deployment template.

Here's an example of installing this chart with persistent storage on openshift:

helm -n chartmuseum-dev install chartmuseum . --set persistence.enabled=true --set persistence.storageClass=myStorageClass --set securityContext.enabled=false --set volumePermissions.enabled=false