northd: Add support for disabling vxlan mode. #816

Job	Run time
osx clang --disable-ssl	3m 34s
linux gcc --disable-ssl	2m 18s
linux clang --disable-ssl	2m 25s
linux gcc test -500	13m 29s
linux gcc test 501-1000	8m 51s
linux gcc test 1001-	4m 25s
linux clang test sanitizers -300	9m 39s
linux clang test sanitizers 301-600	12m 16s
linux clang test sanitizers 601-900	7m 24s
linux clang test sanitizers 901-1200	5m 54s
linux clang test sanitizers 1201-	5m 15s
linux gcc test -ljemalloc -500	12m 42s
linux gcc test -ljemalloc 501-1000	9m 36s
linux gcc test -ljemalloc 1001-	4m 29s
linux clang test -ljemalloc -500	12m 21s
linux clang test -ljemalloc 501-1000	9m 42s
linux clang test -ljemalloc 1001-	4m 49s
linux gcc system-test -100	10m 53s
linux gcc system-test 101-200	16m 10s
linux gcc system-test 201-	18m 22s
linux clang system-test sanitizers -100	12m 16s
linux clang system-test sanitizers 101-200	17m 52s
linux clang system-test sanitizers 201-	19m 37s
linux x86 gcc --disable-ssl	2m 36s
	3h 46m 55s

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