[airgap] Fix values processing for 2A extensions

[eromanova]

This PR fixes the values processing for 2A extensions:

1. Run helm template before getting values for extensions:

${HELM} template "${template}" --show-only "${k0sconfig}" | $YQ e "${extensions_path}.charts[] | select(.chartname == \"*$chartname*\") | .values" > ${name}-values.yaml

instead of

grep -vw "{{" $template | $YQ e "${extensions_path}.charts[] | select(.chartname == \"*$chartname*\") | .values" > ${name}-values.yaml

3. Rename templates_dir -> TEMPLATES_DIR

Closes #654