distro/rhel7: update distribution implementation

Update the implementation of the distro.Distro interface to match the
one in RHEL 8, 9, and Fedora.  The main change is that the runner is a
runner.Runner and not a string.

The runner name is now rhel79 (changed from rhel7).  This is
functionally equivalent based on osbuild's runner version fallback
logic.

test/data/manifests/rhel_7-x86_64-azure_rhui-boot.json

@@ -40,7 +40,7 @@
     "pipelines": [
       {
         "name": "build",
-        "runner": "org.osbuild.rhel7",
+        "runner": "org.osbuild.rhel79",
         "stages": [
           {
             "type": "org.osbuild.rpm",

test/data/manifests/rhel_7-x86_64-qcow2-boot.json

@@ -51,7 +51,7 @@
     "pipelines": [
       {
         "name": "build",
-        "runner": "org.osbuild.rhel7",
+        "runner": "org.osbuild.rhel79",
         "stages": [
           {
             "type": "org.osbuild.rpm",

test/data/manifests/rhel_7-x86_64-qcow2_customize-boot.json

@@ -121,7 +121,7 @@
     "pipelines": [
       {
         "name": "build",
-        "runner": "org.osbuild.rhel7",
+        "runner": "org.osbuild.rhel79",
         "stages": [
           {
             "type": "org.osbuild.rpm",