gcp-vpc-move-vip: Fix sort for list of dicts in python3

python2 sorts a list of dicts of {'ipCidrRange': <alias>} correctly by
default. python3 fails with a TypeError:

TypeError: '<' not supported between instances of 'dict' and 'dict'

Fix this by using the key parameter of sorted(). python2 also supports
this.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>