Oversubscription in Thin Provisioning

We can adopt the similar mechanism introduced in Cinder:
https://github.com/openstack/cinder-specs/blob/master/specs/kilo/over-subscription-in-thin-provisioning.rst

Proposed new parameters:

provisioned_capacity: It is the apparent allocated space indicating how much capacity has been provisioned.

max_over_subscription_ratio: It is the float representation of the over subscription ratio when thin provisioning is involved. It is a ratio of provisioned capacity over total capacity. Default ratio is 1.0, meaning provisioned capacity cannot exceed the total capacity. Note that this ratio is per backend or per pool depending on driver implementation. Add option in manila.conf.

thin_provisioning_support = True or False (True for sparse files) (Add sparse_file option in manila.conf)
thick_provisioning_support = True or False

Existing parameter:
reserved_percentage (reserved_share_percentage in manila.conf)

Generic driver:
    Currently reports 'infinite' for total_capacity_gb and free_capacity_gb.
    Using Cinder quota API we can get some information:

$ cinder quota-usage 575976ef9ec54883b94fb8ae29e5e815+-----------------------+--------+----------+-------+
| Type | In_use | Reserved | Limit |
+-----------------------+--------+----------+-------+
| backup_gigabytes | 5 | 0 | 1000 |
| backups | 5 | 0 | 10 |
| gigabytes | 9 | 0 | 1000 |
| gigabytes_lvmdriver-1 | 9 | 0 | -1 |
| snapshots | 1 | 0 | 10 |
| snapshots_lvmdriver-1 | 1 | 0 | -1 |
| volumes | 8 | 0 | 10 |
| volumes_lvmdriver-1 | 8 | 0 | -1 |
+-----------------------+--------+----------+-------+

$ cinder quota-show 575976ef9ec54883b94fb8ae29e5e815 +-----------------------+-------+
| Property | Value |
+-----------------------+-------+
| backup_gigabytes | 1000 |
| backups | 10 |
| gigabytes | 1000 |
| gigabytes_lvmdriver-1 | -1 |
| snapshots | 10 |
| snapshots_lvmdriver-1 | -1 |
| volumes | 10 |
| volumes_lvmdriver-1 | -1 |
+-----------------------+-------+

Tenant running the command is "demo" and the id belongs to "demo". Will investigate more on how to report capacity for the generic driver.

get_pools is Admin API so we can't use it.

Link to summit discussion:
https://etherpad.openstack.org/p/YVR-manila-liberty-thin-provisioning