Discover and store unknown/un-templated ONU default MIB

For any ONU that does not have an existing template and is subsequently Reset and MIBUpload/MIBUploadNext, optionally store the state of the MIB just after this sequence is complete. This will allow for ONU that do not have a template to have a template created for them by an administrator.

The overall design for MIB Templating can be found here: https://drive.google.com/open?id=1qAAKijnSXfFO2zLOJHLjmsUpBJBeSJfh7y8cupZfwwA

Currently this MIB is printed by a DEBUG log statement. Rather than this the ONU's newly reset MIB should be stored in etcd for later manual retrieval and creation into a template.

Only one instance of this discovered MIB is needed for any ONU type defined by the same rules as MIB Templating (ONU Vendor/ONU Equipment ID/ONU Software Version). If this key is found by a lookup in etcd, then that a discovered default onu MIB instance already exists and there is no need to store it again.

This feature should be configurable and optionally enabled.

Discovered ONU MIB Storage Path Prefix:

service/voltha/omci_mibs/discovered_types/

Example Instances:

service/voltha/omci_mibs/discovered_types/BRCM/BVM4K00BRA0915-0083/5023_020O02414
service/voltha/omci_mibs/discovered_types/ALCL/BVMGR00BRAXS020XA/3FE47059BFHB21

With this feature in place administrators can then retrieve these unknown ONU MIB types from etcd and construct a template.