add an integration test to install core and some fixes

Note that it may not be possible to create an fsimage to use as a core
installation source (haven't tried, tbh) but I have upcoming changes to
use a disk image as installation source.

examples/answers/core.yaml

@@ -0,0 +1,24 @@
+#source-catalog: examples/sources/core.yaml
+Source:
+  source: ubuntu-core
+Welcome:
+  lang: en_US
+Refresh:
+  update: no
+Keyboard:
+  layout: us
+Zdev:
+  accept-default: yes
+Network:
+  accept-default: yes
+Proxy:
+  proxy: ""
+Filesystem:
+  guided: yes
+  guided-index: 0
+UbuntuPro:
+  token: ""
+InstallProgress:
+  reboot: yes
+Drivers:
+  install: yes

examples/sources/core.yaml

@@ -0,0 +1,10 @@
+- description:
+    en: This test source provides a "Ubuntu Core" base install
+  id: ubuntu-core
+  locale_support: none
+  name:
+    en: Ubuntu Server
+  path: ubuntu-core.squashfs
+  size: 530485248
+  type: fsimage
+  variant: core

subiquity/models/oem.py

@@ -39,6 +39,7 @@ def __init__(self):
         self.install_on = {
             "server": False,
             "desktop": True,
+            "core": False,
         }
 
     def make_autoinstall(self) -> Dict[str, Union[str, bool]]:

subiquity/server/server.py

@@ -214,11 +214,12 @@ def get_installer_password_from_cloudinit_log():
     "filesystem",
     "kernel",
     "keyboard",
-    "mirror",
     "network",
     "proxy",
     "source",
-    })
+    },
+    desktop={'mirror'},
+    server={'mirror'})
 
 POSTINSTALL_MODEL_NAMES = ModelNames({
     "drivers",