ovs: Fix OVS path mismatch when building from source with OVN

compile_ovs was called with --localstatedir=/var, causing all compiled OVS binaries to hardcode /var/run/openvswitch as the default runtime directory. However, the ovn_agent plugin derives OVS_RUNDIR from OVS_PREFIX as /usr/local/var/run/openvswitch, creating a path mismatch that makes ovs-vsctl unable to reach the running ovsdb-server.

Additionally, start_new_ovs was unconditionally starting OVS during the install phase. When OVN is the backend, ovn_agent:_start_ovs later starts its own OVS instance with a separate database, but the orphaned ovs-vswitchd from start_new_ovs still holds the kernel datapath, preventing bridge creation.

Fix both issues:

• Remove the --localstatedir=/var override so the compiled default matches OVS_RUNDIR (autotools defaults localstatedir to ${prefix}/var).
• Skip start_new_ovs when OVN services are enabled, since _start_ovs handles the full OVS lifecycle.
• Fix the hardcoded DB path in start_new_ovs to match the compiled --prefix=/usr/local sysconfdir.