Fixed undef variables problems in ssh_config.erb

376b873:
Undefined variable were not properly identified in ssh_config.erb. This created wrong lines in ssh_config:
ForwardAgent undef
ForwardX11 undef
ServerAliveInterval undef

Now undefined variables are recognized and lines will get suppressed if so.

78ed132:
removes duplicate documentation from the modules code. This docu is included in README.me already.