exoscale: add an option to specify a SSH user

Thanks to the last commit, it is now possible to spawn different images,
notably Debian and CoreOS. The default user in those images are not
"ubuntu". Let the user choose the appropriate user ("core" for CoreOS
and "debian" for Debian).

Signed-off-by: Vincent Bernat <Vincent.Bernat@exoscale.ch>

docs/drivers/exoscale.md

@@ -25,7 +25,8 @@ Options:
 -   `--exoscale-disk-size`: Disk size for the host in GB (10, 50, 100, 200, 400).
 -   `--exoscale-image`: Image template (eg. ubuntu-14.04, ubuntu-15.10).
 -   `--exoscale-security-group`: Security group. It will be created if it doesn't exist.
--   `--exoscale-availability-zone`: exoscale availability zone.
+-   `--exoscale-availability-zone`: Exoscale availability zone.
+-   `--exoscale-ssh-user`: SSH username, which must match the default SSH user for the used image.
 
 If a custom security group is provided, you need to ensure that you allow TCP ports 22 and 2376 in an ingress rule. Moreover, if you want to use Swarm, also add TCP port 3376.
 
@@ -41,3 +42,4 @@ Environment variables and default values:
 | `--exoscale-image`              | `EXOSCALE_IMAGE`             | `ubuntu-15.10`                    |
 | `--exoscale-security-group`     | `EXOSCALE_SECURITY_GROUP`    | `docker-machine`                  |
 | `--exoscale-availability-zone`  | `EXOSCALE_AVAILABILITY_ZONE` | `ch-gva-2`                        |
+| `--exoscale-ssh-user`           | `EXOSCALE_SSH_USER`          | `ubuntu`                          |

drivers/exoscale/exoscale.go

@@ -37,6 +37,7 @@ const (
 	defaultDiskSize         = 50
 	defaultImage            = "ubuntu-15.10"
 	defaultAvailabilityZone = "ch-gva-2"
+	defaultSSHUser          = "ubuntu"
 )
 
 // GetCreateFlags registers the flags this driver adds to
@@ -88,6 +89,12 @@ func (d *Driver) GetCreateFlags() []mcnflag.Flag {
 			Value:  defaultAvailabilityZone,
 			Usage:  "exoscale availibility zone",
 		},
+		mcnflag.StringFlag{
+			EnvVar: "EXOSCALE_SSH_USER",
+			Name:   "exoscale-ssh-user",
+			Value:  defaultSSHUser,
+			Usage:  "Set the name of the ssh user",
+		},
 	}
 }
 
@@ -109,7 +116,11 @@ func (d *Driver) GetSSHHostname() (string, error) {
 }
 
 func (d *Driver) GetSSHUsername() string {
-	return "ubuntu"
+	if d.SSHUser == "" {
+		d.SSHUser = defaultSSHUser
+	}
+
+	return d.SSHUser
 }
 
 // DriverName returns the name of the driver
@@ -130,6 +141,7 @@ func (d *Driver) SetConfigFromFlags(flags drivers.DriverOptions) error {
 	}
 	d.SecurityGroup = strings.Join(securityGroups, ",")
 	d.AvailabilityZone = flags.String("exoscale-availability-zone")
+	d.SSHUser = flags.String("exoscale-ssh-user")
 	d.SetSwarmConfigFromFlags(flags)
 
 	if d.URL == "" {