Add description to service config in admin command

[thuningxu]

$ ./cherami-admin cfg --help
NAME:
   cherami serviceconfig - Manage service configs. Supported service names, config keys and usage are:
"cherami-controllerhost", "numPublisherExtentsByPath",      comma-separated list of "prefix=extent_count"
"cherami-controllerhost", "numConsumerExtentsByPath",       comma-separated list of "prefix=extent_count"
"cherami-controllerhost", "numRemoteConsumerExtentsByPath", comma-separated list of "prefix=extent_count"
"cherami-controllerhost", "activeZone",                     the active zone for multi-zone consumers
"cherami-controllerhost", "failoverMode",                   "enabled" or "disabled"
"cherami-storehost",      "adminStatus",                    if set to anything other than "enabled", will prevent placement of new extent on this store
"cherami-storehost",      "minFreeDiskSpaceBytes",          integer, minimum required free disk space in bytes to place a new extent
"cherami-outputhost",     "messagecachesize",               comma-separated list of "destination/CG_name=value" for message cache size
      

USAGE:
   cherami serviceconfig command [command options] [arguments...]

[coveralls]

Coverage increased (+0.04%) to 68.651% when pulling 259928e on admin into 6442730 on master.

[datoug]

actually there should be only one activeZone

[datoug]

enabled or disabled

[coveralls]

Coverage decreased (-0.4%) to 68.397% when pulling 02de19f on admin into d81c1fa on master.

[coveralls]

Coverage decreased (-0.03%) to 68.332% when pulling a4a06f5 on admin into 76665c4 on master.