Lower case switch vendor OUIs in vendors.json never match the switches OUIs

When an unknown switch OUI must be added to cord, a file /etc/maas/switches/vendors/vendors.json must be created.

The file has the following format (i.e.):
[

{ "prefix":"48:0f:cf", "vendor":"Hewlett Packard Enterprise", "provision":true }

]

If the OUI prefix format is lower case, the modification doesn't have any effect, causing new switches to don't be provisioned.

To reproduce on CORD 2.0:

• sudo mkdir -p /etc/maas/switches/vendors
• sudo vi /etc/maas/switches/vendors/vendors.json
• Append the json reported in the first lines of the ticket
• sudo vi /etc/maas/automation-compose.yml
  -> under the SWITCHQ section add "/etc/maas/switches/vendors:/vendors"
  -> under the environment section add "SWITCHQ_VENDORS_URL=file:///vendors/vendors.json":
• docker-compose -f /etc/maas/automation-compose.yml log switchq
• docker-compose -f /etc/maas/automation-compose.yml log SWITCHQ
• docker-compose -f /etc/maas/automation-compose.yml logs switchq

A user should be able to insert the OUI prefix upper or lower case, having anyway the right effect.