[autotest] Refactor readonly_connection

The way readonly connections were used used 
some monkey patching and manual connection handling.

Code gets easier to maintain if Django manages all the
database connections and no monkeypatching is done.
This is what this CL achieves.

TEST=Ran suites.

Change-Id: I2761af10f5172bb3558e7dbd7c1f1875f7a7cb51
Reviewed-on: https://chromium-review.googlesource.com/223393
Tested-by: Jakob Jülich <jakobjuelich@chromium.org>
Reviewed-by: Dan Shi <dshi@chromium.org>
Reviewed-by: Fang Deng <fdeng@chromium.org>
Commit-Queue: Jakob Jülich <jakobjuelich@chromium.org>
7 files changed