Investigate whether HTTPRule parser allows invalid templates

As part of #2825, @oyvindwe mentioned that the HTTPRule parser may allow templates that will cause the OpenAPIv2 generator to generate invalid paths. Specifically:

Does the HttpRule parser also allow other path templates outside the specification? Specifically : before a variable? The reason I am asking is that the the joining of the path in the generator assumes that only the last part can start with :.

We should investigate whether this is the case, and if so, probably patch the openapiv2 generator to remove this assumption, as introducing errors for these templates now is a backwards incompatible change.

Related to #2824.