createhdds: add a disk image for use by a 'support server'

Summary:
this will be used for tests like iSCSI and NFS which need a
server end, but where (unlike e.g. FreeIPA) we don't want set
up of the server end to be considered a test and run on the
compose being tested, but instead we just consider the server
end a 'test asset' and expect it to be as reliable as possible.

For now we just set it up to provide an iSCSI target, as that's
what I want to write a test for first.

Note: I was initially writing the actual iSCSI target config
file as part of image creation, but that turns out to be a bit
tricky because we can't safely write a relative or absolute path
for the source file into the commands file. We'd have to instead
allow specifying file injections in the JSON, similar to how we
do for guestfs images, then construct an absolute path in the
Python code using SCRIPTDIR, but it was easier to just write the
file in the openQA test instead, so I did that.

Test Plan:
check that creating the support image works (and if
you're being really thorough, boot it and check the package is
there).

Reviewers: jskladan, garretraziel

Reviewed By: garretraziel

Subscribers: tflink

Differential Revision: https://phab.qadevel.cloud.fedoraproject.org/D883

hdds.json

@@ -156,6 +156,12 @@
             },
             "size" : "20G",
             "imgver": "2"
+        },
+        {
+            "name" : "support",
+            "releases" : {
+                "-1" : ["x86_64"]
+            }
         }
     ],
     "renames" : []

support.commands

@@ -0,0 +1,4 @@
+root-password password:weakpassword
+update
+selinux-relabel
+install scsi-target-utils