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
make sure we obtain the correct type of logger object (EasyBuildLog, not FancyLogger)
correct import statements for modify_env and read_environment (import from easybuild.tools.environment now)
fix typo
add unwanted_env_vars easyconfig parameter, as opposed to using EasyBlock class variable
Merge pull request #673 from wpoely86/change_env_vars

Unset & restoring environment variables functions for eb's