Add userdata support to `scw-boot-tools` (fix #80)

[moul]

Here is an example with the public API: https://github.com/scaleway/scaleway-cli/blob/master/pkg/cli/x_userdata.go

You should use http://169.254.42.42/ user_data/ without token instead

[moul]

You can checkout scaleway/scaleway-cli#167

[moul]

root@dev-scw-cli:~/scaleway-cli# ./scw -D _userdata local
DEBU[0000] curl -X GET -H "X-Auth-Token: " -H "Content-Type: application/json" http://169.254.42.42//user_data
toto
toto2
root@dev-scw-cli:~/scaleway-cli# ./scw -D _userdata local toto
DEBU[0000] curl -X GET -H "X-Auth-Token: " -H "Content-Type: application/json" http://169.254.42.42//user_data/toto
42
root@dev-scw-cli:~/scaleway-cli# ./scw -D _userdata local toto=43
DEBU[0000] curl -X PATCH -d 43 -H "X-Auth-Token: " -H "Content-Type: text/plain" http://169.254.42.42//user_data/toto
toto
root@dev-scw-cli:~/scaleway-cli# ./scw -D _userdata local toto=
DEBU[0000] curl -X DELETE -H "X-Auth-Token: " -H "Content-Type: application/json" http://169.254.42.42//user_data/toto