Manila CI jobs create lots of shares and for Generic driver it means
lots of Cinder volumes. After merge of [1] to Cinder project, we started
having NoValidHostFound errors in Cinder logs, it changed default
oversubscription ratio to 1.0 from 20.0. But Manila CI requires it
be bigger than 1.0, so set it explicitly to Cinder's default value 20.0
that is used by all other driver but not LVM.
[1] I5a214462b573d00d26086512821b4f8cc1db4fc3
Change-Id: If0a6e535b92b1471572e1de98e3b44ab6a2c65c9
Closes-Bug: #1540947