Add support for building snap packages (#174)

* Add support for building snap packages

This branch makes a few changes to support running the exo binary from a
snap package (https://snapcraft.io).


* Don't build with -j2

Turns out our build was running the build step (go build) and the clean
step (go clean) in parallel.

Make sure snapcraft runs Makefile targets with -j1 (like on a default
local machine).

* Make store-mandated changes

Also add comments to the snap file pointing to the forum-based process.

.gitignore

@@ -4,3 +4,5 @@ exo
 manpage
 contrib
 cli
+*.snap
+*.xdelta3

cmd/root.go

@@ -203,6 +203,11 @@ func initConfig() {
 		gConfigFolder = path.Join(usr.HomeDir, ".config", "exoscale")
 	}
 
+	// Snap packages use $HOME/.exoscale (as negotiated with the snap store)
+	if _, snap := os.LookupEnv("SNAP_USER_COMMON"); snap {
+		gConfigFolder = path.Join(usr.HomeDir, ".exoscale")
+	}
+
 	if gConfigFilePath != "" {
 		// Use config file from the flag.
 		viper.SetConfigFile(gConfigFilePath)

snap/snapcraft.yaml

@@ -0,0 +1,45 @@
+name: exoscale-cli
+base: core18
+version: git
+summary: The Exoscale command line tool
+description: |
+  The Exoscale command line tool.
+
+  This snap allows you to use the "exo" command once installed, your
+  gateway to the Exoscale platform.
+
+  Usage: sudo snap install exoscale-cli; exo config; exo vm list
+
+grade: stable # must be 'stable' to release into candidate/stable channels
+confinement: strict # use 'strict' once you have the right plugs and slots
+
+plugs:
+  # This allows us to read/write in $HOME/.exoscale/*
+  # It was granted via a forum post and is autoconnected.
+  # Do not change the plug name.
+  dot-exoscale:
+    interface: personal-files
+    write:
+      - $HOME/.exoscale
+
+apps:
+  # The app has an alias to "exo" that was granted via forum post.
+  exoscale-cli:
+    command: exo
+    plugs:
+      - network
+      - home
+      - dot-exoscale
+
+parts:
+  exo:
+    # See 'snapcraft plugins'
+    disable-parallel: True
+    plugin: make
+    source: .
+    artifacts:
+      - exo
+    build-snaps:
+      - go
+    build-packages:
+      - git