Search for env variable to find avahi library.

If avahi is installed in non-standard location use env variable
AVAHI_PATH to specify location of libraries i.e.:

AVAHI_PATH=/lib/libavahi-client.so ./cf-agent -B

If not specified cf-agent will look for libraries in default locations.
Add unit test for automatic bootstrap.

Test cases covered:

1. No hubs found.
2. One hub found.
3. Multiple hubs found.
4. Avahi error occured.