Support resignature of vmfs snapshots

[smazurov]

Added ability (and a test that skips unless there is an available snapshot) to resignature vmfs snapshots. Basically this flow

[vmwclabot]

@smazurov, you must sign our contributor license agreement before your changes are merged. Click here to sign the agreement. If you are a VMware employee, read this for further instruction.

[dougm]

Looks good, thanks @smazurov

[smazurov]

@dougm typo addressed, nice catch!

[dougm]

Thanks @smazurov . Were you able to sign the CLA?

[vmwclabot]

@smazurov, your company's legal contact has approved your signed contributor license agreement. It will also be reviewed by VMware, but the merge can proceed.

[vmwclabot]

@smazurov, VMware has approved your signed contributor license agreement.