Verify MTU is valid for ipsec_site_connection

Verify MTU is valid for ipsec_site_connection

Since the check relies on the vpnservice subnet to determine whether
this is IPv4 ro IPv6, we must check in the plugin. Test is done at
create/update time and ensures that the MTU is equal to or greater
than the minimum allowed values, which are set to 68 for IPv4
minimum and 1280 for IPv6, respectively.

Refactored code to allow reuse of create and update test functions,
by allowing tests to override some settings, and to provide a dict
of changed items (for update).

bug 1219489

Change-Id: I3e62ef786d3a02c761903a15d546ee8758c0bf7f