Read the image access server URL from the config file

This pull request adds support for reading the image access server url from the config file.  This makes testing easier as gns3-ias can be run locally.

Example setting to go in config file:
gns3_ias_url=http://127.0.0.1:8888

There is also a code change in base_cloud_ctrl.py that supports the FULLY_OPERATIONAL instance booting state.  More states are added in later pull requests.