Add FSAS_PROVISIONING_LANPORT_IDX handling

[wilkowskia]

Summary

Makes the provisioning lanport index configurable via the new FSAS_PROVISIONING_LANPORT_IDX environment variable, instead of always using the hardcoded constant. This mirrors the existing pattern used for FSAS_SSH_MAX_ATTEMPTS.

Changes

• Added a new getProvisioningLanportIdx() helper in fm.go that reads the FSAS_PROVISIONING_LANPORT_IDX env var:
  • Falls back to the default (PROVISIONING_LANPORT_IDX = 3) when the variable is unset.
  • Validates the value is a non-negative integer; on parse errors or negative values it logs the issue and falls back to the default.
  • Declared as a var function so it can be overridden in tests.
• Used getProvisioningLanportIdx() when populating the provisioning subnet's LanportIdx in populateCreateMachineRequest.
• Added TestGetProvisioningLanportIdx covering unset, valid positive, zero, non-numeric, and negative inputs.

Behavior

FSAS_PROVISIONING_LANPORT_IDX	Result
unset	default (3)
valid non-negative int (e.g. 5, 0)	uses the provided value
non-numeric (e.g. abc)	default (3)
negative (e.g. -1)	default (3)