redfishpower: refactor internals to use plugs

[chu11]

Problem: Redfishpower was originally written with only "hosts" in mind, however the correct terminology is for powerman devices to operate with "plugs". In most cases, redfishpower "hosts" are simply the same names as the "plugs".

Internally refactor everything to operate on "plugs". This refactor includes:

• adding a global "plugs" that holds all plugs
• adding a global "plug_map" that maps plugs to hostnames
• initially define all "plugs" to be the same name as the hostnames
• map plug names to hostnames via a new plug2host() function
• all output is based on plugnames instead of hostnames
• rename all functions and variables (e.g. stat_cmd_host() is now stat_cmd_plug()).

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This PR now precedes #157

[chu11]

ok, re-worked this PR (and the PR series) to use a broken out "plugs" module. Now all plugs data is in the struct plugs_data struct and many of the functions are now in the "plugs" module.

[garlick]

Looking at what comes next, it might simplify the code a little if this were structured like a true class, e.g. with create/destroy functions. Then you wouldn't need to keep a flag to remember if plugs were initialized (just set to object to NULL) or have a "destructor" that removes plugs while iterating the hostlist. Just a thought.

[chu11]

re-pushed, making the plugs module more like a class. i'll rebase follow on PRs later.