Requiring shared directory for remote execution of tasks about sos

[gaow]

Agreed, considering most of our user cases so far are the simpler case of completely local or remote.

[BoPeng]

One problem with this approach is that the $HOME directory on different systems can be different (although there can be shared mounts). It is then necessary to allow the specification of ~/.sos/tasks to a directory that is shared. Because it can be a bad idea to mix jobs executed on different hosts, the tasks directory should better be host-specific.

For backward compatibility, let us keep tasks to home directory. In this way, sos installed on remote servers does not need to be updated.

We need to

1. Confirm if input file is on one of the shared directories.
2. Confirm if workdir etc are on one of the shared directories.

[BoPeng]

Done, with a new version of sos released. #1536