Non-disruptive backup

Non-disruptive backup

This patch adds support for non-disruptive backup for
volumes in 'in-use' status as follows:

Adds a force flag in create backup API when backing up
an 'in-use' volume.

For the default implementation in volume/driver.py:
* Create a temporary volume from the original volume
* Backup the temporary volume
* Clean up the temporary volume

For the LVM driver:
* Create a temporary snapshot
* Obtain local_path for the temporary snapshot
* Backup the temporary snapshot
* Cleanup the temporary snapshot

Attach snapshot will be implemented in another patch.

Partial-implements blueprint non-disruptive-backup
Change-Id: I915c279b526e7268d68ab18ce01200ae22deabdd