Handle trailing slash in Mender client's server URL configuration

When building with Yocto Project, if you set

MENDER_SERVER_URL = "https://hosted.mender.io/"

(note the trailing slash), the client will not be able to connect to the server, and fail with the following error messages:

Nov 14 16:32:39 raspberrypi3 mender[270]: level=info msg="State transition: authorize [Sync] -> authorize-wait [Idle]" module=mender
Nov 14 16:32:39 raspberrypi3 mender[270]: time="2017-11-14T16:32:39Z" level=error msg="authorize failed: transient error: authorization request failed: unexpected authorization status 404" module=state

If you remove the trailing slash, to make it like this:

MENDER_SERVER_URL = "https://hosted.mender.io"

it works as expected.

This is too fragile and will lead to many configuration and diagnostics issues.

This YP variable is likely just copied directly into the client configuration, so the fix should go into the client.

Acceptance criteria

• The Mender client is able to connect to the server both if there is a trailing slash in the server URL and not