MountPath in VolumeMount is a required field. Removing the omitempty.

Removing omitempty for MountPath in VolumeMount definition.
Please also update the version-specific instances:
https://github.com/GoogleCloudPlatform/kubernetes/blob/master/pkg/api/v1beta1/types.go#L173
https://github.com/GoogleCloudPlatform/kubernetes/blob/master/pkg/api/v1beta2/types.go#L147
https://github.com/GoogleCloudPlatform/kubernetes/blob/master/pkg/api/v1beta3/types.go#L256