Automatic addition of `themes_config_file_by_host: {}` disables themes

[cat-bro]

After updating this role to version 0.10.19 our themes have disappeared, along with the option to select a theme in user preferences.

In our galaxy.yml file we have

    themes_config_file: /mnt/galaxy/config/themes_conf.yml
    themes_config_file_by_host: {}

We can get our themes back by commenting out themes_config_file_by_host.

[mira-miracoli]

@hexylena I am not sure if there is a possibility to omit these keys when default.yml is not a jinja template