Add pvc namer interface

[sanjid133]

For some providers, there have limitations on volume name. e.g. for Linode, maximum length of the volume name is 32 characters (Length of default namer is 40 characters). So, this interface will help those providers to use own namer.

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[rootfs]

/assign @jsafrane

[wongma7]

Please follow the pattern here https://github.com/kubernetes-incubator/external-storage/pull/612/files#diff-16daa5c3f02eee06add7f1af0a3fa901L234 to make it an optional arg e.g. something like

func Namer(namer Namer) func(*ProvisionController) error {
 	return func(c *ProvisionController) error {
 		if c.HasRun() {
 			return errRuntime
 		}
 		c.namer = namer
 		return nil
 	}
}

this is just so not everyone has to add controller.NewDefaultNamer(), to their args should they update. Thanks & apologies for months-long delay before review

Otherwise the code lgtm

I am actually unsure about whether we should let authors name the pvc however they want or enforce a suffix -PVC UID, I will need to check spec

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