Allow operator to disable Swift ring building.

The default method for updating Swift rings - such as adding new
nodes to existing rings - is to do the update manually and then
distribute the new rings to all Swift nodes.  At the moment Heat
builds rings by default when a Swift node is brought up.  We do
not want this to happen when Heat is being used to bring up new
Swift nodes for inclusion in an existing overcloud Swift cluster.
This patch checks for a piece of meta-data swift.ring-build,
which defaults to True.  If instead the metadata is set to False
or F then Swift rings will not be built.  The metadata can be
set to False or F by an operator at the heat command-line, in
particular at heat update to an existing overcloud stack.

Change-Id: Ia2544d560b20d895b3b0b19bc7c91542ad58509d
Merge "Allow operator to disable Swift ring building."