tools: add snmp oid validation.

An oid is valid iff 2 <= size() <= 128 and if the first
two sub-identifiers are less than 3 and 40 respectively.

Emtpy values are not valid. a missing value already generates an
exception when getting property.

Change-Id: I2469bfac312eb38dad7de093c8fefe144f75bbb2
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1073498
Signed-off-by: Mooli Tayer <mtayer@redhat.com>