Use COMMON_GIT_MIRROR

**Ticket:** []()

**Background:**
Some times ,because of the GFW or worse network,people in China can not use github services.

 I open this pull  to make sure we can switch our local edx  repo mirrors.

**Studio Updates:**  None.

**LMS Updates:** None

**Test:**
[travis](https://travis-ci.org/eduStack/configuration/builds/43984602)