Backward compatibility for sticky

[ldez]

Description

if sticky=true -> cookieName="_TRAEFIK_BACKEND"

for new custom cookieName feature:

• must add stickiness=true
• add cookieName="MyCookie"

---

summary:

Options	Result
onlystickiness=true	activate sticky session and use a 6 chars cookie name (abbrev sha1)
onlysticky=true	(deprecated) activate sticky session and use the old cookie name _TRAEFIK_BACKEND
stickiness=true and sticky=true	same as stickiness=true
stickiness=true and cookieName=foobar	activate sticky session and use foobar as the cookie name
onlycookieName=foobar	do nothing
sticky=true and cookieName=foobar	same as sticky=true, doesn't use the cookieName value

---

@MichaelErmer could you retry with this image ldez/traefik:sticky?

Related to #2251, #2238, #2232

[MichaelErmer]

Docker Swarm works perfectly now! Thanks a lot @ldez

[lvnilesh]

Docker Swarm works using ldez/traefik:sticky

[nmengin]

LGTM

[emilevauge]

thanks a lot @ldez 👏
LGTM

[gabrielfsousa]

is web gui working ? for me no :(
sorry, this dont work on swarm :(

test run in non service/swarm, and works, and sticky works now

[juliens]

LGTM