Fix apicast replace_path to use remove

[guicassolato]

What this PR does / why we need it:

It changes the routing policies injected into the proxy config for the backends of a product so they use the replace_path rule with remove_first instead of with replace.

Which issue(s) this PR fixes

Porta is currently generating proxy configs for products containing backends with paths with the following attribute in the corresponding injected routing policy:

"replace_path": "{{original_request.path | replace: '/the-path', '/'}}"

This is causing apicast to route the incoming traffic, for example, from <public-domain>/the-path/actual-backend-path, to <private-domain>//actual-backend-path. The correct backend endpoint would be <private-domain>/actual-backend-path.

We should use instead in the routing policy injected for the backend:

{{original_request.path | remove_first: '/the-path' }}

Closes THREESCALE-3593

Verification steps

1. Create a product containing at least 2 backends
2. Generate the proxy config
3. Confirm in the json file of the proxy config that the routing policies injected in the chain for the backends with path include a replace_path rule in the following format:

   "replace_path": "{{original_request.path | remove_first: '/the-path'}}"
   

Example of part of a real proxy config generated using this PR

Special notes to the reviewer

Related to 3scale/APIcast#1122