Change "peer_cidrs" field to MultiIpField

Change "peer_cidrs" field from IPField to MultiIpField.
This will help to choose a list of subnets for IPSec Site Connection
instead of one subnet only.
Some tests are also added.

Fixes bug 1257253

Change-Id: Ib68bd0ad7373694fba4a04f9486bb31410eb322f
Merge "Change "peer_cidrs" field to MultiIpField"