load configuration only from one directory

[wkloucek]

Description

We've changed the configuration file loading behavior and are now only loading
configuration files from ONE single directory. This directory can be set on
compile time or via an environment variable on startup (OCIS_CONFIG_DIR).

We are using following configuration default paths:

• Docker images: /etc/ocis/
• Binary releases: $HOME/.ocis/config/

Related Issue

• Should be merged after [full-ci] Refactor storage configuration to make it similar to other oCIS configuration #3569
• Unblocks [full-ci] ocis init & remove default secrets #3551, because we can now write the initiated config file to the SINGLE known location. Before this PR, we wouldn't know where to write the config file.
• Part of improve systemd docs and simplify config file arithmetics #3480, which tries to add also flags to specify config files directly

Motivation and Context

Make config file loading less confusing for users.

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests only (no source changes)

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation ticket raised:

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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