ShellCommandFencer should expose info about source

Currently the HA fencing logic in ShellCommandFencer exposes environment variable about only the fencing target. i.e. the $target_* variables as mentioned in this document page).

But here only the fencing target variables are getting exposed. Sometimes it is useful to expose info about the fencing source node. One use case is would allow source and target node to identify themselves separately and run different commands/scripts.