teuthology/task/install/valgrind.supp: add suppression for Boost.Thread

teuthology/task/install/valgrind.supp: add suppression for Boost.Thread

Boost.Thread passes `tls_destructor` to `pthread_key_create()` in hope
to free the allocated memory stored in TLS key `current_thread_tls_key`,
but neither Boost.Thread nor us uses `pthread_exit()` for calling the
cleanup functions. and Boost.Thread is against `pthread_exit()`, see [0,1].

but Boost.Thread offers a preprocessor macro to define a global variable
whose destructor calls `tls_destructor()`, but per [2], this macro is
not defined by default. and per [3], this macro could cause assertion
failure in Boost. so it might be advisable to not define it, even we
could do so in BuildBoost.cmake.

and since this `Leak_StillReachable` leak is a one-shot thing. i am
adding it to the suppression file.

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[0] https://www.boost.org/doc/libs/1_68_0/doc/html/thread/thread_management.html#thread.thread_management.tutorial.native_from._code__phrase_role__identifier__pthread_exit__phrase___code__posix_limitation
[1] https://svn.boost.org/trac10/ticket/5013
[2] https://svn.boost.org/trac10/ticket/3926
[3] https://svn.boost.org/trac10/ticket/12049

Fixes: http://tracker.ceph.com/issues/22052
Signed-off-by: Kefu Chai <kchai@redhat.com>