openUBMC: Huawei Default SNMPv3 USM key

Huawei's openUBMC ships a default SNMPv3 USM key that uses one value for both authentication and privacy. An attacker who holds the key (recoverable from the open-source build) can authenticate SNMPv3 to, and decrypt SNMPv3 traffic from, any unit that still carries the shipped key. The build customization manifest/build/customization/default_password.lua sets AuthenticationKey == EncryptionKey == 7df2f29369e3ac3245b2c9d46b67a0ecd055197e41808a34ef325db2f6422581 for the iBMC product build, and the community seed account/manufacture/default_password.lua sets both to 7a2f5b86f20233ec7b7bef49643fbf34d86d0cbf6f1f8f5c52d4f30d4536938b. These are shipped constants, not per-unit values, and they are not rotated by the Administrator password reset, so they survive account provisioning. This is especially dangerous considering that SNMP access is mapped to full device administration privileges, including system reset and firmware upgrades.