VPP-69: Create fib or bridge domain with arbitrary ID

It appeared to me that it might be usefull
to allow users to create fibs or bridge domains without
a complete knowledge of the current used IDs.

These changes define fib and bridge domain constructors
when the provided ID is ~0. In such a case, an unused ID is
used to create a new fib or bridge domain.

Change-Id: Iaba69a023296e6d17bdde45980f9db84832a3995
Signed-off-by: Pierre Pfister <ppfister@cisco.com>