Refactor lxc/completion.go

[kadinsayani]

As we've expanded and improved our shell completions, code duplication and room for improvement has increased across lxc/completion.go.

Refactorings and improvements to consider:

• Extract common logic into separate functions;
• Expand usage of map[string]struct{} to improve lookup times;
• Audit use of completion directives and ensure consistency across CLI commands and sub commands (ShellCompDirectiveNoFileComp and ShellCompDirectiveNoSpace);
• Cacheing metadata API response (see CLI: Improvements to lxc config and lxc profile shell completions #14542 (comment) and CLI: Improvements to lxc config and lxc profile shell completions #14542 (comment));
• Improving image cacheing.