Support for local_settings.py settings override file

[sol1-matt]

This PR configures local_settings.py.

This is required for the deployment of the branch plugin as this file must exist before the migrations are run if the plugin is enabled. You can't otherwise use this role if you want the branch plugin.

• The role now automatically tries to copy from {{ playbook_dir }}/files/netbox/local_settings.py, files/netbox as local_settings.py is a generic name that could be used by other plays or roles.
• Errors are ignored on the copy in place of testing if the files exists or a var being set
• The symlink creation tests for the existence of local_settings.py in the shared director. If a user add's local_settings.py to the shared directory manually the role will still use it.
• The role doesn't have a way to remove the local_settings.py from the shared directory, if the user wants to remove the local_settings.py they can remove it from shared.

[lae]

Please see comments.

for own reference,
background on what the local_settings.py file is and why one would use it: netbox-community/netbox#16127

[sol1-matt]

I've committed changes but still need to test them.

[sol1-matt]

@lae I've tested this now and updated the readme, the last commit for this is 282202a.
It is ready for merge.

[lae]

Just for future reference is defined is used to test if a variable is defined. Since we set an empty default it'll still error like below:

TASK [lae.netbox : Copy NetBox local_settings.py into shared (ignore if it doesn't exist)] ***************************************************************************************************
fatal: [10.17.0.122]: FAILED! => {"changed": false, "msg": "src (or content) is required"}
...ignoring

TASK [lae.netbox : Symlink/Remove NetBox local_settings.py file into/from the active NetBox release] *****************************************************************************************
fatal: [10.17.0.122]: FAILED! => {"changed": false, "msg": "src file does not exist, use \"force=yes\" if you really want to create the link: /srv/netbox/shared/local_settings.py", "path": "/srv/netbox/current/netbox/netbox/local_settings.py", "src": "/srv/netbox/shared/local_settings.py"}

We have a couple other role variables that only get used if they're defined, but just left commented out in the documentation and defaults file, so I applied that pattern here.

[lae]

I just noticed: congrats on being the 100th PR on this project!