gh-61: Fix context scripts locking
gh-61: Fix context scripts locking

Use directory lock instead of file lock. Ensure the environment is
cleaned (e.g. the CONTEXT ISO is unmounted) and lock is cleaned
via bash trap in case of failure.

Closes #61.

fasrc/addon-context-linux,puppetmaster/addon-context-linux,OpenNebula/addon-context-linux,puppetmaster/addon-context-linux,fasrc/addon-context-linux,OpenNebula/addon-context-linux