Allow specifying specific servers, not just groups, in custom crons

[jamesmontalvo3]

Currently we have the following, which allows specifying server_type for custom crons.

{% if custom_crons is defined  %}
#
# The following jobs are custom definitions from this meza-instance's config
#
{% for cron in custom_crons %}{% if inventory_hostname in groups[cron.server_type] %}
{{ cron.time }} {{ cron.job }}

{% endif %}{% endfor %}

This allows a configuration like:

custom_crons:
  - server_type: app-servers
    time: "* * * * *"
    cron: echo "this is a test" >> /tmp/test.txt

It would probably better to specify servers and allow it to be either a single server or a group or servers (or perhaps multiple servers, such that these are all possible:

  servers: app-servers
  ...
  servers: app-servers[0]
  ...
  servers: [ 'app-servers', 'parsoid-servers' ]
  ...
  servers: [ 'app-servers', parsoid-servers[0] ]

The above may not work to determine the difference between strings and lists, so maybe need to do:

  servers: "{{ groups['app-servers'] }}"
  ...
  servers: "{{ groups['app-servers'][0] }}"
  ...
  servers: "{{ groups['app-servers'] + groups['parsoid-servers'] }}"
  ...
  servers: "{{ groups['app-servers'] + [ groups['parsoid-servers'][0] ] }}"