Improve api-frontend error handling on invalid X-PacketFence-Tenant-Id header

[satkunas]

Describe the bug
GET /api/v1/config/realm/DEFAULT

with header:

X-PacketFence-Tenant-Id: undefined (or any non numeric value)

produces

Sep 11 13:48:53 packetfence api-frontend[15216]: t=2020-09-11T13:48:53-0400 lvl=warn msg="Impossible to parse X-PacketFence-Tenant-Id undefined into a valid number, error: strconv.Atoi: parsing \"undefined\": invalid syntax" pid=15216 request-uuid=0e8a9641-f457-11ea-8d1b-080027d5be68