[RFE] Manual option to select Active Geo-replication worker

[aravindavk]

New Geo-replication option to manually choose Active workers. From each distribute group the first brick is selected as Active worker and all other workers will be Passive. This approach has multiple problems:

• If the worker belonging to the first brick of a distribute group goes to Faulty, then Geo-rep will not select other workers as Active.
• If the worker is supposed to be Passive but first brick of some other distribute group exists in the same node, then multiple workers belonging to same distribute group will be Active.

Meta Volume option (https://docs.gluster.org/en/main/Administrator-Guide/Geo-Replication/#configurable-options) is introduced in Geo-rep that makes automatic switching when a worker goes to Faulty. This is good, but causes lot of fluctuations and repeat syncing.

Introduce a new option to manually select the Active workers. By default, select the first node of a distribute group as Active.

gluster volume geo-replication <primary-volume> <secondary-host>::<secondary-volume> \
    config active-workers <brick-paths>

Example:

gluster volume geo-replication vol1 server1b::vol2 \
    config active-workers "server1:/data/gfs/vol1/b1,server4:/data/gfs/vol1/b4"

• If the brick path not specified for a distribute group, then select the first brick by default. No need to specify the brick path if we want first brick to be active.
• Raise error if multiple brick paths from same distribute group is specified.