Fixed the calculation of available capacity test CSI plugin.
Fixed the calculation of available capacity test CSI plugin.

The size of existing volumes should be substracted away from the
available capacity. Otherwise, the total capacity will keep increasing
every time the plugin restarts.

Review: https://reviews.apache.org/r/64665/

andschwa/mesos,asamerh4/mesos,asamerh4/mesos,jpeach/mesos,chhsia0/mesos,gsantovena/mesos,Gilbert88/mesos,asamerh4/mesos,Gilbert88/mesos,asamerh4/mesos,kaysoky/mesos,abudnik/mesos,kaysoky/mesos,Gilbert88/mesos,kaysoky/mesos,abudnik/mesos,dforsyth/mesos,gsantovena/mesos,andschwa/mesos,dforsyth/mesos,Gilbert88/mesos,asamerh4/mesos,kaysoky/mesos,jpeach/mesos,abudnik/mesos,gsantovena/mesos,reneploetz/mesos,andschwa/mesos,abudnik/mesos,kaysoky/mesos,reneploetz/mesos,dforsyth/mesos,chhsia0/mesos,kaysoky/mesos,abudnik/mesos,gsantovena/mesos,andschwa/mesos,jpeach/mesos,kaysoky/mesos,andschwa/mesos,dforsyth/mesos,asamerh4/mesos,abudnik/mesos,jpeach/mesos,Gilbert88/mesos,jpeach/mesos,reneploetz/mesos,reneploetz/mesos,chhsia0/mesos,chhsia0/mesos,andschwa/mesos,Gilbert88/mesos,gsantovena/mesos,dforsyth/mesos,gsantovena/mesos,reneploetz/mesos,reneploetz/mesos,chhsia0/mesos,gsantovena/mesos,chhsia0/mesos,andschwa/mesos,dforsyth/mesos,reneploetz/mesos,jpeach/mesos,Gilbert88/mesos,abudnik/mesos,asamerh4/mesos,dforsyth/mesos,chhsia0/mesos,jpeach/mesos