fix NameError: @@debug is not initialized if debug mode is off

Remove @@debug in favor of the 'DEBUG' env var which is used in the
debug method in barclamp_mgmt_lib

/opt/dell/bin/barclamp_install.rb:190: uninitialized class variable @@debug in Object (NameError)
	from /opt/dell/bin/barclamp_install.rb:123:in `each'
	from /opt/dell/bin/barclamp_install.rb:123
Merge pull request #1990 from mapleoin/debug-fail

fix NameError: @@debug is not initialized if debug mode is off