Add binary install directory

[guimaluf]

Binaries are currently droped in /usr/local/bin

This commit add node_exporter_binary_install_dir variable where user can
change the binary destination to /opt/bin for example.

[paulfantom]

It was a conscious decision to not provide such option in any of ansible role located in @cloudalchemy. /usr/local/bin is the correct place for those binaries and it strictly adheres to man hier(7).

What is the reasoning behind providing this as a feature?

[guimaluf]

The main reason is that in the cluster I'm working with the /usr/local/bin is a mounted directory which I don't have permissions to change.

I'm aware that the correct hierarchy resides on /usr/local/bin that manner I keep the default as it is.
I could remove the task where the directory is created and let just as an option.

I don't mean to change the default behaviour. Just make it customizable.

[paulfantom]

After I merged it, I remembered one problem with this approach here and in previous designs. Always someone proposed to include a task responsible for creating a directory where node_exporter binary would be placed. This, in turn, had huge impact on managing directory permissions in default installations as we are by default making this role responsible for managing /usr/local/bin ownership (in

ansible-node-exporter/tasks/install.yml

Lines 2 to 7 in d08e7c2

	- name: create node_exporter binary install directory
	file:
	path: "{{ _node_exporter_binary_install_dir }}"
	state: directory
	owner: root
	group: root

).

This task will be removed in future PR and if anyone wants to create a directory, then he/she needs to do it on a playbook level as managing system directories is out of scope for this role.