Allow overriding default location with environment variable

[wcooley]

I would like to be able to override the default path with an environment variable, like GENDERS_FILE (or maybe just GENDERS?).

I use genders on my laptop and desktop and maintain my genders file in a git repo. Unlike our servers, my laptop and desktop are not under configuration management, so it's messy to keep /etc/genders updated -- I have to symlink to my checkout in my home directory, manually override the path whenever I run nodeattr or manually copy the updated file into place.

I can also imagine scenarios where an individual user might want to use genders/pdsh and, while she might be able to convince her sysadmins to install the packages, having them maintain a shared database is more problematic. Being able to use a personal copy would be a good first step to bringing the tool into a new environment.

I can also imagine potential security implications with allowing users to override the location, if genders/nodeattr/pdsh are used in scripts run with elevated privileges, so it would probably be necessary to include a compile-time flag to disable this feature for sites that might have such requirements.

[chu11]

I can also imagine scenarios where an individual user might want to use genders/pdsh and, while she might be able to convince her sysadmins to install the packages, having them maintain a shared database is more problematic

I think this is a good use case example.