Fix OMP_NUM_THREADS setting

[muupan]

• Move os.environ['OMP_NUM_THREADS'] = '1' before import chainer to make it work
• Add it to async examples under examples/gym.

[muupan]

Current master

$ time python examples/ale/train_a3c_ale.py 4 breakout --steps 1000
...
real    0m16.618s
user    0m28.334s
sys     1m28.136s

This PR

$ time python examples/ale/train_a3c_ale.py 4 breakout --steps 1000
...
real    0m5.907s
user    0m19.049s
sys     0m0.275s

Numpy config

python -c "import numpy; numpy.__config__.show()"
blas_mkl_info:
  NOT AVAILABLE
blis_info:
  NOT AVAILABLE
openblas_info:
    libraries = ['openblas', 'openblas']
    library_dirs = ['/usr/local/lib']
    language = c
    define_macros = [('HAVE_CBLAS', None)]
blas_opt_info:
    libraries = ['openblas', 'openblas']
    library_dirs = ['/usr/local/lib']
    language = c
    define_macros = [('HAVE_CBLAS', None)]
lapack_mkl_info:
  NOT AVAILABLE
openblas_lapack_info:
    libraries = ['openblas', 'openblas']
    library_dirs = ['/usr/local/lib']
    language = c
    define_macros = [('HAVE_CBLAS', None)]
lapack_opt_info:
    libraries = ['openblas', 'openblas']
    library_dirs = ['/usr/local/lib']
    language = c
    define_macros = [('HAVE_CBLAS', None)]

[toslunar]

LGTM