Add new 'infra_only' Makefile target and Landing Zone VM support to
dev-scripts, enabling creation of baremetal-like test infrastructure
without deploying an OpenShift cluster.

Changes:

1. New 'infra_only' Makefile target
   - Runs 'requirements' and 'configure' steps only
   - Stops before cluster deployment (build_installer, ironic, etc.)
   - Useful for testing external deployment tools

2. Landing Zone VM support
   - Added NUM_LANDINGZONE environment variable (default: 0)
   - Configurable specs: LANDINGZONE_MEMORY (8192 MB), LANDINGZONE_DISK
     (60 GB), LANDINGZONE_VCPU (4)
   - Landing Zone VM has dual network access (BMC and cluster networks)
   - Worker VMs use cluster network only when NUM_LANDINGZONE > 0

3. NetworkManager connection handling fix
   - Fixed nmcli error when cluster network is managed by libvirt
   - Suppress error if NetworkManager connection doesn't exist:
     sudo nmcli con del ${BAREMETAL_NETWORK_NAME} 2>/dev/null || true
   - Prevents make infra_only from failing in CI pipelines

4. Documentation
   - Added "Infrastructure-Only Mode" section to README
   - Added "Deploying with Landing Zone VMs" section to README

Use case: Testing external deployment tools (e.g., GoRI Lab) that need
infrastructure without cluster deployment.

Backward compatibility: All existing targets work unchanged. New variables
default to 0 (no Landing Zone VMs).

Allow users to configure a custom libvirt storage pool name instead of
the hardcoded "oooq_pool" default. This is useful for environments that
require specific storage pool naming conventions or want to use existing
pools.

Changes:
- Add LIBVIRT_VOLUME_POOL environment variable with default "oooq_pool"
- Pass libvirt_volume_pool to ansible-playbook in 02_configure_host.sh
- Document the new variable in config_example.sh