Add config option for share volume fs type

During the creation of the share server with the generic driver, on
block storage provided by cinder 'mkfs' is called. This commit makes the
used filesystem selectable. ext4 and ext3 are supported for now.

Change-Id: Ibd39ff356e984aef304cc16065e280caa610d3e3
Closes-Bug: #1357768
Merge "Add config option for share volume fs type"