Topology: YAML topology files and ordered gateway tiers

[thiell]

Implements the design discussed in #374 (see the design comment). Closes #374.

YAML topology files

• New topology.yaml file syntax: routes as a list of gateways/targets objects. Selected by file extension (.yaml/.yml), so --topology works with both syntaxes; INI topology.conf remains fully supported.
• Unlike INI, the same gateways may appear in several routes: one gateway pool can reach several target groups, with symmetric failback — the core need from the PR Tree: support optional route weights in topology.conf (#374) #678 review.
• topology.yaml is preferred over topology.conf when both exist at the same config location.
• TopologyParser becomes a thin facade over per-format backends; graph building and validation are unchanged and format-agnostic. The INI code path is untouched and still covered by the existing tests.
• PyYAML is imported lazily, only when a YAML topology is loaded (same pattern as YAML group files). Reviewer question (HLD §8): keep it optional like this, or make PyYAML a hard dependency? Note install_requires already pulls PyYAML for pip installs, so this mostly matters for distro packages.

Ordered gateway tiers

• In YAML, the gateways of a route may be a list of entries with descriptive attributes, in the style of modern YAML tools such as lnetctl: nodes (node set, required), weight (load share within a tier, default 1) and tier (failover rank, default 1). A tier is used only when all gateways in lower tiers are unreachable:

routes:
  - gateways:
      - nodes: gw1
      - nodes: gw2
        tier: 2             # gw1 default, gw2 failover
    targets:  rio[100-199]
  - gateways:
      - nodes: gw2
      - nodes: gw1
        tier: 2             # gw2 default, gw1 failover
    targets:  rio[200-299]

• A plain list of node sets (gateways: [gw1, gw2]) is a load-shared pool, equivalent to gateways: gw[1-2] — failover is always explicit with tier, and the entry form keeps the schema extensible for future per-gateway attributes.
• Within a tier, load is shared with least-connections as today, weighted by weight (integers >= 1). Weight 0 is rejected: a standby gateway is a higher tier, so there is only one mechanism.
• Tiers apply to all traffic routed through the pool, including targets deeper in the tree.
• Resolved tiers ride the pickled TopologyTree; trees from older versions are still accepted, and the wire protocol is unchanged.

Docs and tests

• config.rst/clush.rst document the YAML syntax, quoting rules (@group values must be quoted), precedence and tiers; conf/topology.yaml.example added.
• New tests/TreeRouterTest.py covers gateway selection: tier order, failover, mutual failback, weighted distribution, pool equivalence of plain lists, multi-level trees, pickled-tree round-trip, pre-1.11 tree compatibility. YAML parser tests added to TreeTopologyTest, including INI/YAML tree equivalence.

Supersedes PR #678 (now a draft): the INI [weights] section is not kept, and its weighted least-connections selection is reused as the within-tier policy.