Proxy configuration with parameterized backend uri fails to load

Azure Functions Core Tools (2.0.1-beta.37)

#### Repro steps
1. Create a proxies.json configuration file with a parameterized route
2. Pass the parameter to the host name part of the backend

```json
{
"$schema": "http://json.schemastore.org/proxies",
"proxies": {
"createorder": {
"matchCondition": {
"methods": [
"POST"
],
"route": "/tenants/{tenantName}/people"
},
"backendUri": "https://{tenantName}.thedomain.com/people/",
"debug": true
}
}
}
```
3. `func host start`

#### Expected behavior

Func host to load configuration and route calls to backend using specified template

#### Actual behavior
Func host fails to process the configuration
`[2018-09-12 19:13:14] Error processig configuration of Function Proxies`

Start with the proxies.json configuration shown in the issue and run `func host start` using a parameterized backend URI. Trace the configuration-processing failure for the `{tenantName}` host parameter. Done means the host loads successfully and routes the request to the specified backend template.

Written by the indexing model from the issue text.