Enforce Bridge Domain ID range to match 24-bit VNI range

Enforce Bridge Domain ID range to match 24-bit VNI range

Enforce bridge domain ID range to allow a maximum value of 16M
which matches the range of 24-bit VNI used for virtual overlay
network ID.
Fix "show bridge-domain" output to allow full 16M BD ID range
to be displayed using 8-digit spaces.

Change-Id: I80d9c76ea7c001bcccd3c19df1f3e55d2970f01c
Signed-off-by: John Lo <loj@cisco.com>