[#3278] Updated ARM

[#3278] Updated ARM

Added ChangeLog entry, updated ARM, and changed
enable-monitoring to false by default.

doc/sphinx/arm/hooks-perfmon.rst
    Added a good deal more information

src/hooks/dhcp/perfmon/perfmon_config.cc
src/hooks/dhcp/perfmon/perfmon_mgr.cc
    enable-monitoring now defaults to false. Users must
    explicitly enable it.

src/hooks/dhcp/perfmon/tests/perfmon_config_unittests.cc
src/hooks/dhcp/perfmon/tests/perfmon_mgr_unittests.cc
    Updated tests