Support over subscription in thin provisioning

Support over subscription in thin provisioning

This patch adds support for over subscription in thin provisioning.
The following changes are proposed:

* A configuration option "max_over_subscription_ratio" will be
  introduced.
* Driver reports the following capacities and ratios:
  * provisioned_capacity
  * max_over_subscription_ratio
    * Driver can use the newly added configuration option to report
      this ratio or it can decide what ratio to report itself.
      The value of this ratio is depending on driver implementation
      and will be reported together with other capabilities and
      capacities by the driver.
  * reserved_percentage
    * Note: This is an existing parameter reported by the driver.
    * Currently it is measured against the free capacity. In this
      patch, it will be changed to measure against the total
      capacity in the filter scheduler.
* Driver also reports the following capabilities:
  * thin_provisioning_support (True or False)
  * thick_provisioning_support (True or False)
* Scheduler will use the above new capabilities reported by the
  driver to make decisions when choosing a backend.

For more details, please see Cinder spec:
https://review.openstack.org/#/c/129342/12/specs/kilo/
over-subscription-in-thin-provisioning.rst

Implements: blueprint over-subscription-in-thin-provisioning
Change-Id: I176a691a4e25bbdc7c4d598628acf2543b2c7ac6