vimconn.py: Better documentation.

Unify new_network method for all vimconns.
Add new parameter persistem_info at constructor.
New method check_vim_connectivity.

Change-Id: Ie2c7900fe12fd5ae01ef2bb4891b79a0f9173285
Signed-off-by: tierno <alfonso.tiernosepulveda@telefonica.com>