Allow to override jgroups subnet

[guidograzioli]

New optional variable:

Variable	Description
keycloak_jgroups_subnet	Override the subnet match for jgroups cluster formation; if not defined, it will be inferred from local machine route configuration

allows to statically override the <subnet-match value=".."/> for the <interface name="jgroups">.

If left undefined (default), the subnet match is computed as:

(ansible_default_ipv4.network + '/' + ansible_default_ipv4.netmask) | ansible.utils.ipaddr('net')

The above won't work when instances are isolated (on a a.b.c.d/32 network)