Bootable filter for listening volumes through cinder-cli.

Currently CLI does not support bootable filter option to select bootable volumes from existing volumes.
But cinder document specifies bootable as default option for volume_query.

This is possible on Horizon end to select only bootable volume's list while launching instance.

Refer https://bugs.launchpad.net/cinder/+bug/1524450 for same.

Also in cinder-API documents, it is mentioned that bootable is default option to go ahead with selecting volumes.
https://bugs.launchpad.net/openstack-manuals/+bug/1536845

So, its good time to support same behavior for cinder client as well.

We can get bootable/non-bootable results like this.
  cinder list --fields bootable | grep False
  cinder list --fields bootable | grep True

But its good to have this kind of functionality on server end than client side.

Accumulative motivations for this feature are:

1. To list only bootable volumes from CLI to use with instances.

2. May be its preferable(usability point of view) to use filter than searching through grep and all at user/admin level.

3. Also, In good softwares, linux level commands are not exposed to user to use on CLI prompt.
Need to check with openstack if this kind of behavior exists or it's all open to user.

4. Its kind of overhead to handle this kind of things at client end.