Support for only installing select services on a node

[nmaludy]

This Ansible module supports only installing/running select services by specifying the st2_services variable. However, when doing so it causes issues in various cases because, for example, things like st2ctl reload --register-all can only be run on a st2actionrunner node where the pack content is available. If you're on a node just running st2api this command is going to fail.

This change does the following:

• Implements some simple when conditions for various handlers to only restart services when they are necessary
• Fixes st2ctl on the node to only contain the specified services in its COMPONENTS variable, so that st2ctl command still work as expected! :)
• Fixes a small deprecation warning on the pip PR i did yesterday:

  [DEPRECATION WARNING]: Invoking "pip" only once while using a loop via squash_actions is deprecated. Instead of using a loop to 
  supply multiple items and specifying `name: "{{ item }}"`, please use `name: '{{ st2_python_packages }}'` and remove the loop. 
  This feature will be removed from ansible-base in version 2.11. Deprecation warnings can be disabled by setting 
  deprecation_warnings=False in ansible.cfg.

@armab i'm thinking about also adding some when conditions to the st2smoketests where appropriate. Could you help me figure out what the conditions should be there?

Thanks!

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