Changes

asterisk-master: Fetch required branch directly

When I added the Dockerfile I initially wrote it in a 2 step fetch &
build, to try to avoid recompiling the whole thing every time some patch
from one of the devs was pushed to our building branch.
This, whoever, creates problems sometimes due to how the build system
for pjproject is generated when building asterisk, so let's drop the
2stage setup to avoid problems fetching from different branches, etc.

Change-Id: Id9fbee9cdb8f9387e0daefd7925df2542d384a91