Enable Fedora 19 on DevStack

Fedora uses a special PROMPT_COMMAND for screen terminal types,
which are the default with DevStack.  The PROMPT_COMMAND interacts
in a negative way with the -t and -X and -p flags, causing DevStack
not to work.  To solve this problem, this patch forces PROMPT_COMMAND
to default to /bin/true, triggering no changes to the window title
used within screen.  Also this patch enables F19 as a non-FORCE
distribution.

Change-Id: I2414d9e28dd95b69272e132163b29ed83f73b2f6
Fixes: bug #1196340
lib/cinder comment

The default volume backing size is ~10G now.
Changing the comment to be more future-proof by including
the shell variable name.

Change-Id: I27879b240f3f8348fa069853de18e47871ce00d2
Merge "Enable Fedora 19 on DevStack"
Merge "lib/cinder comment"