northd: Don't generate IPv6 prefix delegation flows if not configured. #3273

Job	Run time
osx clang --disable-ssl	2m 24s
linux rpm fedora	6m 57s
dpdk gcc	5s
Prepare container	1m 53s
linux gcc --disable-ssl	2m 14s
linux clang --disable-ssl	2m 28s
linux gcc test -500	14m 7s
linux gcc test 501-1000	10m 18s
linux gcc test 1001-	5m 48s
linux clang test sanitizers -300	11m 0s
linux clang test sanitizers 301-600	14m 7s
linux clang test sanitizers 601-900	9m 15s
linux clang test sanitizers 901-1200	7m 31s
linux clang test sanitizers 1201-	7m 0s
linux gcc test -ljemalloc -500	14m 54s
linux gcc test -ljemalloc 501-1000	10m 37s
linux gcc test -ljemalloc 1001-	6m 11s
linux gcc system-test-dpdk dpdk -100	10m 49s
linux gcc system-test-dpdk dpdk 101-200	17m 43s
linux gcc system-test-dpdk dpdk 201-	18m 56s
linux gcc system-test-userspace -100	9m 41s
linux gcc system-test-userspace 101-200	16m 0s
linux gcc system-test-userspace 201-	17m 48s
linux gcc system-test -100	12m 45s
linux gcc system-test 101-200	18m 33s
linux gcc system-test 201-	25m 36s
linux clang system-test sanitizers -100	15m 45s
linux clang system-test sanitizers 101-200	19m 56s
linux clang system-test sanitizers 201-	27m 32s
linux x86 gcc --disable-ssl	2m 13s
	5h 40m 6s

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