MINOR: cli: make "show env" accessible via master CLI without enabling debug

MINOR: cli: make "show env" accessible via master CLI without enabling debug

Before this patch, we have need to put the master CLI in debug mode to be able
to issue 'show env' command for the master process. Output of this command is
handy even for the master process context, as it allows to control its
environment variables, which could be used/modified in the 'global' section.

So, let's provide in 'show env' command structure the level ACCESS_MASTER.
This allows to see and to access this command in master CLI without putting it
in debug mode.