Implement `check` command

[keisukefukuda]

Implement a command that checks environments of remote nodes

• mpienv check-env ENVVAR ... - Checks environmental variables of remote nodes
• mpienv check-python [-m module] - Checks python versions of remote nodes. -m also checks if module versions via pip.
• mpienv check-mpi - Checks MPI versions of remote nodes.
• mpienv check-command - Checks if a command exists in PATH variable in each remote node.
• mpienv check-file - Checks if a certain file exists in each remote node.
• mpienv check-all - Checks everything possible