fabrics: Allow discover params to come from a conf file

fabrics: Allow discover params to come from a conf file

Allow the user to just run "nvme discover" or "nvme connect-all"
in case it finds a default /etc/nvme/nvmf_disc conf file.

We allow multiple discovery addresses by iterating over the
lines of the file and executing a discover (with or without
connect) for each line. We allow newlines and '#' prefixed comments.

The return value is or'ed on all discover attempts.

In order to minimize some parsing code, I just convert the
file line into an (argc, argv) pair and feed it to argconfig_parse()
which dictates that the file lines are identical to what one would
pass nvme discover <params>. I'm open to better ideas.

Signed-off-by: Sagi Grimberg <sagi@grimberg.me>