Add VolumeSnapshot.created attribute

[allardhoeve]

One very important aspect of snapshots is when they are made, especially when used as historical backups. The field in VolumeSnapshot.extra that is used to represent this data varies between OS, EC2, GCE and Rackspace. Also, EC2 boasts a datetime value in extra, while OpenStack and derivatives use a plain string.

This PR adds a created attribute to VolumeSnapshot, that is a parsed date. This unified all implementations.

[allardhoeve]

Added to aid introspection in all sorts of IDEs and ipython.

[Kami]

The behavior is not exactly the same. Previously those variables were only instance variables, now they are also class variables which could have undesired consequences in some cases (https://stackoverflow.com/questions/207000/python-difference-between-class-and-instance-attributes).

I would prefer to keep it as it was. As far as introspection goes -I need to dig into it, but at least as far as helping with types go - you can help IDE's by adding type asserts and docstring annotation (we already do the latter in most places).

[allardhoeve]

I did not know this. My %(deity)s, how often I've used this incorrectly. Thank you.

[allardhoeve]

@Kami

[allardhoeve]

• CloudFrames implements a CloudFramesSnapshot, which has nothing to do with a VolumeSnapshot 😢

[allardhoeve]

To aid introspection in PyCharm.

[Kami]

Besides the class variables change, this PR looks good to me.

[allardhoeve]

Removed instance variables and merged trunk into the branch.