Add tests for start-up config returned when of_config is disabled

Stub function linc_ofconfig:read_startup_without_of_config/0 has been
added that should return start-up configuration for the LINC-Switch
when the of_confnig is enabled.
(Motivated by #139).
Add simple solution for start-up config when of_config is disabled

The linc_ofconfig:read_startup_without_of_config/0 builds the start-up
config for the LINC-Switch the same way when of_config is enabled and
there hadn't been any previous configuration saved.
(Motivated by #139).
Add default port rate in start-up that is needed for configuring queues

If there's no explicit port rate in the sys.config some default value
should be used in the start-up config for the LINC-Switch. Otherwise
module handling port queues will crash.
(Motivated by #139).
Improve generating start-up config for LINC with ofconfig disabled

Now start-up config is generated separately, without using structures
and functions for generating config with ofconfig enabled.
(Motivated by #139).
Refactor linc_ofconfig module

Helper functions were mixed with API. Now it's laid out better.
(Motivated by #139).
Mock inet:getifaddrs/0 in linc_ofconfig_tests

The application cannot start without at least one network interface with
a non-zero MAC address.
Merge pull request #244 from FlowForwarding/of_config-disabled-crash

Of config disabled crash