Change fixed/floating ip clients to return one value and update tests

Add ResponseBodyData for cases like delete_floating_ips_bulk where a
string is returned as the body. The client has to dereference the data
but I think this is a bit cleaner than having some methods return two values.

Partially implements: blueprint clients-return-one-value

Change-Id: I87b092b2dd2a231049fc39312038f2a2d5101028
16 files changed