Add VRRPv3 centisecond advertisement interval support

This commit will add support for centiseconds advertisement interval in
VRRP version 3. To configure an interval in centiseconds, a value between
0.01 and 40.95 can be used.
  advert_int <FLOAT>
Note that an interval in centiseconds is only available in VRRP version 3.

Signed-off-by: Jonas Johansson <jonasj76@gmail.com>