vyos.template: T3664: add a module-level variable for template directory

[dmbaturin]

Add a module-level variable for template directory to enable calling scripts outside of live VyOS systems.

This is a Sagitta version of #3416 .

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe): internal change.

Related PR(s)

#3416, vyos/vyos-build#596

Component(s) name

vyos.template

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly