add docstring for functions

Signed-off-by: Gui Chen <gui.chen@intel.com>
fix pylint

W: 50,0:elf_arch: Dangerous default value ['/bin/bash', '/sbin/init'] as argument
C: 76,4:global_mounts.totuple: Missing docstring
C:126,8:bind_mount: Invalid name "b" for type variable (should match [a-z_][a-z0-9_]{2,30}$)
W:134,4:setup_resolv: No exception type(s) specified
C:168,8:bind_unmount: Invalid name "b" for type variable (should match [a-z_][a-z0-9_]{2,30}$)
W:178,4:cleanup_resolv: No exception type(s) specified
C:175,8:cleanup_resolv: Invalid name "fd" for type variable (should match [a-z_][a-z0-9_]{2,30}$)
C:184,8:kill_processes: Invalid name "fp" for type variable (should match [a-z_][a-z0-9_]{2,30}$)
W:189,8:kill_processes: No exception type(s) specified
C:283,4:chroot.mychroot: Missing docstring

Signed-off-by: Gui Chen <gui.chen@intel.com>