Set LVM driver default overprovisioning ratio to 1.0

Set LVM driver default overprovisioning ratio to 1.0

This introduces a config option, 'lvm_max_over_subscription_ratio',
which allows overriding the global max_over_subscription_ratio.

This is needed because the way we handle configuration with
multi-backend does not allow setting a value which will be applied
to each LVM backend without manually setting it in each backend,
and it would be better to default to a safe overprovisioning
ratio for the LVM driver.  It is often used in smaller deployments
and POCs which may not have all of the capacity monitoring
infrastructure used with other drivers, so overprovisioning should
be an opt-in capability rather than opt-out.

Partial-Bug: #1472803
DocImpact: new config option lvm_max_over_subscription_ratio

Change-Id: I5a214462b573d00d26086512821b4f8cc1db4fc3