Mattermost Resource within Chart Does Not Support Volumes or VolumeMounts

Summary

The bigbang chart for Mattermost does not allow for use of the "volumes" and "volumeMounts" specs built into the "mattermost" resource.

• https://github.com/mattermost/mattermost-operator/blob/master/apis/mattermost/v1beta1/mattermost_types.go#L68
• https://github.com/mattermost/mattermost-operator/blob/master/apis/mattermost/v1beta1/mattermost_types.go#L71

Allow customers to specify volumes and their associated mounts in the values file for the mattermost pods via the Mattermost resource spec.

/cc @BrandenCobb @micah.nagel