models/lbaas.rb: Better error reporting

When raising an exception because of an unexpected HTTP code, include
the code that was received in the exception message.
models/lbaas.rb: delete pool member can return 204

Contrary to the documentation, DELETE on a pool member returns and HTTP
204 rather than an HTTP 202, so accept either code and handle it
appropriately.
controllers/lbaas.rb: Don't request members of new pool

Don't request the members of a pool when we are first creating the pool.
We can assume that the new pool will have no members, and requesting the
members before LBaaS has finished creating the pool will result in an
error.