dhcp6_stateless related tests are failed in RHOSP13

Issue

• dhcp6_stateless related test are failed in RHOSP13
• radvd shows a backtrace like:

 neutron.agent.linux.utils [-] Exit code: -6; Stdin: ; Stdout: ; Stderr: *** Error in `radvd': double free or corruption (!prev): 0x000055b960a794c0 ***
======= Backtrace: =========
/lib64/libc.so.6(+0x81489)[0x7fea7c076489]
radvd(+0x6502)[0x55b95ec2a502]
radvd(+0x3904)[0x55b95ec27904]
/lib64/libc.so.6(__libc_start_main+0xf5)[0x7fea7c0173d5]
radvd(+0x3c9c)[0x55b95ec27c9c]
======= Memory map: ========
55b95ec24000-55b95ec3c000 r-xp 00000000 fc:02 46248928                   /usr/sbin/radvd
55b95ee3b000-55b95ee3c000 r--p 00017000 fc:02 46248928                   /usr/sbin/radvd
55b95ee3c000-55b95ee3d000 rw-p 00018000 fc:02 46248928                   /usr/sbin/radvd
55b960a78000-55b960a99000 rw-p 00000000 00:00 0                          [heap]
7fea74000000-7fea74021000 rw-p 00000000 00:00 0 
7fea74021000-7fea78000000 ---p 00000000 00:00 0 
7fea7bbcc000-7fea7bbe1000 r-xp 00000000 fc:02 251658370                  /usr/lib64/libgcc_s-4.8.5-20150702.so.1
7fea7bbe1000-7fea7bde0000 ---p 00015000 fc:02 251658370                  /usr/lib64/libgcc_s-4.8.5-20150702.so.1
7fea7bde0000-7fea7bde1000 r--p 00014000 fc:02 251658370                  /usr/lib64/libgcc_s-4.8.5-20150702.so.1
7fea7bde1000-7fea7bde2000 rw-p 00015000 fc:02 251658370                  /usr/lib64/libgcc_s-4.8.5-20150702.so.1
7fea7bde2000-7fea7bdee000 r-xp 00000000 fc:02 251658561                  /usr/lib64/libnss_files-2.17.so
7fea7bdee000-7fea7bfed000 ---p 0000c000 fc:02 251658561                  /usr/lib64/libnss_files-2.17.so
7fea7bfed000-7fea7bfee000 r--p 0000b000 fc:02 251658561                  /usr/lib64/libnss_files-2.17.so
7fea7bfee000-7fea7bfef000 rw-p 0000c000 fc:02 251658561                  /usr/lib64/libnss_files-2.17.so
7fea7bfef000-7fea7bff5000 rw-p 00000000 00:00 0 
7fea7bff5000-7fea7c1b7000 r-xp 00000000 fc:02 251658397                  /usr/lib64/libc-2.17.so
7fea7c1b7000-7fea7c3b7000 ---p 001c2000 fc:02 251658397                  /usr/lib64/libc-2.17.so
7fea7c3b7000-7fea7c3bb000 r--p 001c2000 fc:02 251658397                  /usr/lib64/libc-2.17.so
7fea7c3bb000-7fea7c3bd000 rw-p 001c6000 fc:02 251658397                  /usr/lib64/libc-2.17.so
7fea7c3bd000-7fea7c3c2000 rw-p 00000000 00:00 0 
7fea7c3c2000-7fea7c3e4000 r-xp 00000000 fc:02 251658382                  /usr/lib64/ld-2.17.so
7fea7c5da000-7fea7c5dd000 rw-p 00000000 00:00 0 
7fea7c5e1000-7fea7c5e3000 rw-p 00000000 00:00 0 
7fea7c5e3000-7fea7c5e4000 r--p 00021000 fc:02 251658382                  /usr/lib64/ld-2.17.so
7fea7c5e4000-7fea7c5e5000 rw-p 00022000 fc:02 251658382                  /usr/lib64/ld-2.17.so
7fea7c5e5000-7fea7c5e6000 rw-p 00000000 00:00 0 
7ffd38697000-7ffd386b8000 rw-p 00000000 00:00 0                          [stack]
7ffd386e5000-7ffd386e7000 r-xp 00000000 00:00 0                          [vdso]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]