Configure a token for MX access to Incoming API Soledad Server

MX will now write incoming mail to a new soledad server incoming/ entry point instead of writing it directly to couch. Both behaviours will be maintained during some time because of soledad's deprecation policy.

The platform will need to:

• generate a random token that MX will use to auth with soledad.
• write that token to the MX configuration file (see leap_mx#8664 (closed))
• write that token to the Soledad Server configuration file (see soledad#8854 (closed))
• add an stunnel config so MX can access soledad-server node's port 2525 locally. #8866 (closed)
• add the url of the stunnel to the MX config file.

The config file in the MX node (/etc/leap/mx.conf) should have a new section added called [incoming api] with the following content (replace <randomly-generated-token> with the token generated above):

[incoming api]
host=localhost
port=2525
token=incoming:<randomly-generated-token>

The config file in Soledad (/etc/soledad/soledad-server.conf) has a [soledad-server] session that should have the following added:

services_tokens_file = /etc/soledad/services.tokens

And then we need a new file called /etc/soledad/services.tokens with the content (replace <randomly-generated-token> with the token generated above):

incoming:<randomly-generated-token>

Update: fixed the name of the services tokens file.

Update: rephrased token format.

Update: fixed the name of the MX config file.