northd: Respect --ecmp-symmetric-reply for single routes. #3160

Job	Run time
Prepare container	1m 52s
dpdk gcc	5s
linux rpm fedora	7m 8s
osx clang --disable-ssl	2m 20s
linux gcc --disable-ssl	2m 18s
linux clang --disable-ssl	2m 29s
linux gcc test -500	15m 26s
linux gcc test 501-1000	10m 4s
linux gcc test 1001-	5m 49s
linux clang test sanitizers -300	11m 2s
linux clang test sanitizers 301-600	13m 45s
linux clang test sanitizers 601-900	8m 49s
linux clang test sanitizers 901-1200	8m 53s
linux clang test sanitizers 1201-	7m 51s
linux gcc test -ljemalloc -500	15m 4s
linux gcc test -ljemalloc 501-1000	13m 4s
linux gcc test -ljemalloc 1001-	6m 36s
linux gcc system-test-dpdk dpdk -100	11m 33s
linux gcc system-test-dpdk dpdk 101-200	18m 25s
linux gcc system-test-dpdk dpdk 201-	18m 41s
linux gcc system-test-userspace -100	9m 7s
linux gcc system-test-userspace 101-200	15m 46s
linux gcc system-test-userspace 201-	17m 12s
linux gcc system-test -100	16m 45s
linux gcc system-test 101-200	18m 31s
linux gcc system-test 201-	27m 59s
linux clang system-test sanitizers -100	18m 9s
linux clang system-test sanitizers 101-200	20m 38s
linux clang system-test sanitizers 201-	30m 51s
linux x86 gcc --disable-ssl	2m 13s
	5h 58m 25s

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