northd: Don't monitor most of northbound external IDs. #231

Job	Run time
osx clang --disable-ssl	2m 18s
linux rpm fedora	6m 45s
Prepare container	3m 23s
linux gcc --disable-ssl	2m 37s
linux clang --disable-ssl	2m 46s
linux gcc test -300	9m 23s
linux gcc test 301-600	6m 33s
linux gcc test 601- unstable	6m 28s
linux clang test sanitizers -300	12m 27s
linux clang test sanitizers 301-600	9m 26s
linux clang test sanitizers 601- unstable	11m 12s
linux gcc test -ljemalloc -300	9m 43s
linux gcc test -ljemalloc 301-600	6m 29s
linux gcc test -ljemalloc 601- unstable	6m 41s
linux gcc system-test-dpdk dpdk -100	16m 44s
linux gcc system-test-dpdk dpdk 101- unstable	14m 12s
linux gcc system-test-userspace -100	15m 48s
linux gcc system-test-userspace 101- unstable	13m 8s
linux gcc system-test -100	18m 21s
linux gcc system-test 101- unstable	16m 56s
linux clang system-test sanitizers -100	20m 22s
linux clang system-test sanitizers 101- unstable	18m 38s
linux x86 gcc --disable-ssl	2m 50s
	3h 53m 10s

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