LU-9751 snapshot: set PATH for remote zfs commands

LU-9751 snapshot: set PATH for remote zfs commands

It is possible that the remote zfs/zpool commands for Lustre
snapshot are NOT in the remote shell execute/search path. So
needs to set the PATH variable for the remote shell commands.

It is inconvenient for the admin to specify the PATH option
via single lsnapshot command for each Lustre target. So the
patch specifies the remote PATH environment variable as the
the local PATH environment variable. It requires all Lustre
servers to have broadly consistent zfs tools instalation in
such PATH.

It also contains some macro definations for code cleanup.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I2b1ce630d4aad63ab20e6c323f2222dccb51ed6e
Reviewed-on: https://review.whamcloud.com/27999
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>