Allow admin user to get all tenant's floating IPs

When getting floatingips by Nova API, the results will be filtered
with the 'tenant_id'.
So, we can only get the floatingips belonging the tenant of current
context.
When ceilometer invokes novaclient to list floatingips, it will get an
empty list because the tenant is 'service'.
we should allow an admin user to index all tenants's floatingip by adding a
parameter 'all_tenants'.

Part1:
    part1 try to implement this and part2 will remove the unused codes.

Change-Id: I7ab1d5ff463fc29928f6811f846c9e204390a412
Closes-bug: #1262124
Merge "Allow admin user to get all tenant's floating IPs"