make sure clear message is shown as to why extension sanity check fails + minor refactoring
Merge pull request #672 from boegel/sanity_check_msg_exts

make sure clear message is shown as to why extension sanity check fails
Added functions to unset some env vars while building

The EasyBlock class now has a member unwanted_env_vars which has list of
environment variables to unset before building. They are restored after
the build completes.
Moved read_environment to tools/environment

Moved read_environment from tools/utilities to tools/environment.
Moved filetools/modify_env to tools.environment

Moved modify_env from tools/filetools to tools/environment
Updated old deprecated message for read_environment

Moved from config -> utilities -> environment