ipv6: set OtherConfig flag for DHCPv6 stateless subnets

ipv6: set OtherConfig flag for DHCPv6 stateless subnets

In case of DHCPv6 stateless subnets, we should inform DHCP clients about
other configuration values available from DHCP server. This is done by
setting O (other) flag in RAs, which is controlled by AdvOtherConfigFlag
setting in radvd case.

Since radvd configuration file becomes quite complex, migrated its
generation to Jinja2.

Added a basic unit test that checks that flag is set for stateless mode
and not SLAAC. For stateful, it doesn't really matter whether other flag
is set, so no need to expect any value of it.

No more unit tests seem to be needed: conditional prefix generation is
already covered in test_l3_agent, and other statements are common for
all ipv6_ra_modes.

Change-Id: I1ddad3e1f5efce2b6da4ec00b9294e08fe1e85dd
Closes-Bug: #1397022