Support `profile_string` overlay var in releases

Fixes #2159; this is done by force-reloading the handlers to match the
config.
This can create a bit of a funny effect if sys.config specifies an INFO
log level (or lower) is specified. While apps are booted for config
changes before the cth_failonly hook is enabled, supervision and other
application log messages can start being output. They will start being
suppressed once the CT run begins.

This is not a bug, it's a race in instantiation of control and enabling
of log levels. Nothing we can do about that. It might however surprise
people a good bit. If non-default stdout handlers are added, they are
similarly likely to become noisy; specifying a test-specific sys.config
file may be necessary then.

Also includes a bump of cth_readable version, which now checks for
updates to the log formatter on every test output.

Disable the default logger handler in shell if required

See http://erlang.org/doc/man/ct_hooks.html#Module:on_tc_fail-4

To avoid a conflict the Kernel's default handler have to be disabled
before the new default handler is added

    {kernel,
     [{logger,
       [{handler, default, undefined}
       ]}
     ]},
    {my_app,
     [{logger,
       [{handler, default, my_handler, #{}}
       ]}
     ]}

To support this behavior in shell it's necessary to process the
handler-default-undefined tuple.

It's not clear what to do with the simple handler (logger_simple_h)
however.  It's added at the start and removed if a default handler was
added.  Should it be added in reread_logger_config/1 if there's no
default handler?