DevStack plugin: make source dirs configurable

Make source directories of Manila and Manila client configurable such
that those of other OpenStack projects are not affected. So the
developer can conveniently use her own repos with the DevStack setup.
Currently the source directories of Manila and Manila client depend on
$DEST, which is used by other install scripts in the DevStack framework.

Change-Id: I6e430ee2478a097abaab1b78f12d17fed3895eee
Merge "DevStack plugin: make source dirs configurable"