Fix volume capacity reporting

cinder-volume crashes when the decimal point is not represented by a dot
but by a comma instead. This crash happens for example with the de_DE
locale.

This change replaces any comma appearing in the vgs command output with
a dot.

Test case added.

Fixes: bug #1151684
Change-Id: I831055fb7fd206a8560688ec4c25d766e92ee0f0
Merge "Fix volume capacity reporting"