Add no_use_bulkwalkall as an option for discovered devices

[i3149]

Adding a config value to snmp disco to set the no_use_bulkwalkall value for all discovered devices.

Closes #764

When there's a disco config like this

discovery:
    cidrs:
        - 1.2.3.4/28
    ignore_list: []
    debug: false
    ports:
        - 161
    default_communities: []
    use_snmp_v1: false
    default_v3: aws.sm.my-secret-snmp-creds
    add_devices: true
    add_mibs: true
    threads: 4
    replace_devices: true
    check_all_ips: true
    kentik: null
    no_use_bulkwalkall: true

All discovered devices will have no_use_bulkwalkall set to true.

NOTE: if you later on change no_use_bulkwalkall to false in the disco section the discovered devices will continue to have it set to true.

[jryburn]

LGTM