remove code duplication of code dealing with cluster identifier

[mbukatov]

Lot of test functions starts with:

    if cluster_reuse["short_name"]:
        cluster_identifier = cluster_reuse["short_name"]
    else:
        cluster_identifier = cluster_reuse["integration_id"]

and such code duplication is not good.

Instead of copy pasting this code in all tests, value of cluster_reuse fixture should be a object of GlusterCluster class, which will have a method cluster_identifier() which will do what this code does.

[mbukatov]

This could be done with attrs or dataclassess, but latter is available since python 3.7 and we are still on 3.6.