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

Remove @@debug in favor of the standard ruby $DEBUG env var.

/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