Fixes #403 (Github Issue).

This keeps the roles cleaner and more independent of matrix-base,
which may be important for people building their own playbook
out of the individual roles and not using the matrix-base role.

This adds into the Riot config.json the field
  'default_server_config.m.homeserver.server_name'
with, by default, the value of the playbook's 'matrix_domain' variable.

Riot displays this string in its login page and will now say 'Sign in to
your Matrix account on example.org' (the server name) instead of 'Sign
in ... on matrix.example.org' (the server domain-name).

This string can be configured by setting the playbook variable
  'matrix_riot_web_default_server_name'
to any string, so we can make Riot say for example 'Sign in ... on Our
Server'.

This change makes no difference in itself, but is a pre-requisite for
adding the 'server name' config, and the old syntax is deprecated.

Switching to the official image (vectorim/riot-web) should ensure:
- there's less breakage, as it's maintained by the same team as riot-web
- there's fewer actors we need to trust
- we can upgrade riot-web faster, as newer versions should be released
on Docker hub at the same time riot-web releases are made

Fixes #71 (Github Issue).