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/adva_fsp150.yaml

@@ -11,5 +11,4 @@ discovery:
     -
         sysObjectID:
             - .1.3.6.1.4.1.2544.1.12.1.1.
-mib_dir:
-    - adva
+mib_dir: adva