Improve mib_dir setting consistency (#12069)

yaml os discovery now respects the os mib_dir definition
mib_dir now only allows strings, not arrays
changed schema validation to account for that
consolidated dell mibs, no reason for them to be spread out

includes/definitions/ios.yaml

@@ -3,8 +3,7 @@ group: cisco
 text: 'Cisco IOS'
 type: network
 ifXmcbc: true
-mib_dir:
-    - cisco
+mib_dir: cisco
 over:
     - { graph: device_bits, text: 'Device Traffic' }
     - { graph: device_processor, text: 'CPU Usage' }