Add script and generated snapcraft.yaml files (#8096)

This PR adds a proper snapcraft.yaml file for each DNS plugin, and provides a shell script to generate them.

tools/generate_dnsplugins_snapcraft.sh

@@ -0,0 +1,40 @@
+#!/bin/bash
+# Generate the snapcraft.yaml file for all DNS plugins
+set -e
+
+DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null 2>&1 && pwd )"
+CERTBOT_DIR="$(dirname "${DIR}")"
+
+for PLUGIN_PATH in "${CERTBOT_DIR}"/certbot-dns-*; do
+  PLUGIN=$(basename "${PLUGIN_PATH}")
+  DESCRIPTION=$(grep description "${PLUGIN_PATH}/setup.py" | sed -E 's|\s+description="(.*)",|\1|g')
+  mkdir -p "${PLUGIN_PATH}/snap"
+  cat <<EOF > "${PLUGIN_PATH}/snap/snapcraft.yaml"
+name: ${PLUGIN}
+summary: ${DESCRIPTION}
+description: ${DESCRIPTION}
+confinement: strict
+grade: devel
+base: core18
+adopt-info: ${PLUGIN}
+
+parts:
+  ${PLUGIN}:
+    plugin: python
+    source: .
+    constraints: [\$SNAPCRAFT_PART_SRC/snap-constraints.txt]
+    python-version: python3
+    override-pull: |
+        snapcraftctl pull
+        snapcraftctl set-version \`grep ^version \$SNAPCRAFT_PART_SRC/setup.py | cut -f2 -d= | tr -d "'[:space:]"\`
+    build-environment:
+      - EXCLUDE_CERTBOT_DEPS: "True"
+
+slots:
+  certbot:
+    interface: content
+    content: certbot-1
+    read:
+      - \$SNAP/lib/python3.6/site-packages
+EOF
+done