Ensure spec URLs start with a slash.

Signed-off-by: Yorick Peterse <yorickpeterse@gmail.com>
Minor change in slim spec to request template with variable.

Request '/external_vars' to use the '@name' variable along with the 'external_vars.slim' file.

Signed-off-by: Shan Huang <shan.huang@me.com>
Signed-off-by: Yorick Peterse <yorickpeterse@gmail.com>