Fix backward compatibility of environment variables with bool type

[phil-park]

Boolean type environment variable arguments such as GITSYNC_ONE_TIME and GITSYNC_ADD_USER are not backward compatible.

According to the documentation(https://github.com/kubernetes/git-sync/blob/master/v3-to-v4.md), backward compatibility of Version 3's GIT_SYNC_ -> GITSYNC_ should also be supported for boolean type.