Fix crash in crowbar command line tool when no user/password is defined

Arguably it can be seen as a bug in the rubygem, but it's easy to fix
from here.

CROWBAR_KEY not set, will not be able to authenticate!
Please set CROWBAR_KEY or use -U and -P
/opt/dell/bin/barclamp_lib.rb:484:in `run_sub_command': undefined method `tr' for nil:NilClass (NoMethodError)
        from /usr/lib64/ruby/gems/1.8/gems/net-http-digest_auth-1.1.1/lib/net/http/digest_auth.rb:74:in `auth_header'
        from /opt/dell/bin/barclamp_lib.rb:126:in `authenticate'
        from /usr/lib64/ruby/1.8/net/http.rb:543:in `start'
        from /usr/lib64/ruby/1.8/net/http.rb:440:in `start'
        from /opt/dell/bin/barclamp_lib.rb:109:in `authenticate'
        from /opt/dell/bin/barclamp_lib.rb:138:in `get_json'
        from /opt/dell/bin/barclamp_lib.rb:250:in `proposal_list'
        from (eval):1:in `run_sub_command'
        from (eval):1:in `eval'
        from /opt/dell/bin/barclamp_lib.rb:484:in `run_sub_command'
        from (eval):1:in `run_sub_command'
        from /opt/dell/bin/barclamp_lib.rb:488:in `eval'
        from /opt/dell/bin/barclamp_lib.rb:484:in `run_sub_command'
        from /opt/dell/bin/barclamp_lib.rb:488:in `run_command'
        from /opt/dell/bin/barclamp_lib.rb:493:in `main'
        from /opt/dell/bin/crowbar_crowbar:21
Merge pull request #549 from vuntz/fix-crash-on-non-auth

Fix crash in crowbar command line tool when no user/password is defined