Add flag argument to 'cinder-manage config list'

Add flag argument to 'cinder-manage config list'

Unlike other cinder-manage 'list' commands the 'config list'
option doesn't allow users to specify a filter to limit
the output.  This commit adds the ability to specify
the flag the user wishes to display.

If no flag is specified the default behavior is still to
display all the configured flags.  If the flag requested
is not found, cinder-manage reports that the flag was not
found.

(fixes bug 1187137)

Change-Id: I698f4c06d7e93217d8f307a880e0ae40711151c2