Accept existing conf in dynamic_conf

Currently, dynamic_conf doesn't accept an existing conf object and
creates one from scratch every time. This patch adds a `conf` keyword to
both `can_connect` and `dynamic_conf` to re-use the already loaded
options from existing config objects.

This work is needed for the bug mentioned bellow.

Bug: #1372335
Change-Id: I471a33a80504802339accd82e632620d101a23df
Merge "Accept existing conf in dynamic_conf"