Need unit-test for start_esp.py

[qiwzhang]

Any changes to start_esp is not tested in this repo. has to reply on esp e2e test. start_esp.py has 3 parts

1. generate nginx.conf from template by flags,
2. generate server.conf from template by flags
3. remote fetching: token, metadata, service config.

I like to have 1) and 2) unit-test first. 3) may need mock servers.

[lei-tang]

#66 and cloudendpoints/esp#417 are created to add tests for start_esp.