import_tasks instead of include; bringing role up to ansible-prometheus standards; minor changes

[paulfantom]

[minor] modification to introduce some of good ideas seen in #47 and ansible-prometheus

• add ansible_managed info to service file
• use import_tasks instead of include
• specify become: true on role level not on playbook level
• split main.yml into smaller files: install.yml and configure.yml
• use one task for installing dependecies
• use vars files for distro-specific values
• use node_exporter_system_group and node_exporter_system_user which shouldn't be user-defined (defined in vars not in defaults), but simplify role management
• add tags
• add 2 more preflight checks

[SuperQ]

LGTM

[lock]

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