LU-12410 lnet: Implement method to tokenize nidstrings

LU-12410 lnet: Implement method to tokenize nidstrings

The CLI for various lnetctl operations allows the user to specify
multiple, comma separated nidstrings. Implement a common method
for tokenizing nidstrings that can be leveraged by the operations
that require it.

Test-Parameters: trivial
Signed-off-by: Chris Horn <hornc@cray.com>
Change-Id: I2f8ab6d5d9e7c3d5bde3a11b85bdf38fbf6fdf29
Reviewed-on: https://review.whamcloud.com/35505
Reviewed-by: Shaun Tancheff <stancheff@cray.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Petros Koutoupis <pkoutoupis@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>