Change signature of base class (see comment)
Change signature of base class (see comment)

The base driver defines a signature that is not used by any of the drivers implementing `create_volume`.

1. `gandi` does not use the snapshot parameter.
1. `gce` uses the parameter, but accepts either a string or an object or None.
1. `ibm` does not use the snapshot parameter.
1. `opennebula` does not use the snapshot parameter.
1. `profitbricks` breaks the contract entirely.

In this PR both `ec2` and `openstack` implement the signature like this. This matches `gce`, so all drivers using the snapshot functionality act like this new signature.

Signed-off-by: Tomaz Muraus <bb4af54d4692022d34bcf290dffd91d21e083d18@apache.org>

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