Add support for odhcpd DHCP server

[ghost]

Description:

This PR adds support for the odhcp DHCP server in addition to dnsmasq. A new configuration option 'dhcp_software' has been added and allows the user to set the server used (defaults to dnsmasq to not break existing installations).

Pull request in home-assistant.github.io with documentation (if applicable): home-assistant/home-assistant.io#3631

Example entry for configuration.yaml (if applicable):

device_tracker:
  - platform: ubus
    host: gaia.example.org
    username: hass
    password: 1234
    dhcp_software: dnsmasq

Checklist:

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.github.io

If the code communicates with devices, web services, or third-party tools:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• New dependencies have been added to the REQUIREMENTS variable (example).
• New dependencies are only imported inside functions that use them (example).
• New dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
• New files were added to .coveragerc.

If the code does not interact with devices:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• Tests have been added to verify that the new code works.

[homeassistant]

Hi @chemicalstorm,

It seems you haven't yet signed a CLA. Please do so here.

Once you do that we will be able to review and accept this pull request.

Thanks!

[ghost]

I have been running this code for 2 weeks or so using odhcp. I would be glad to have reviews from people using dnsmasq to ensure I didn't break anything when refactoring the code.

Note: I haven't been able to run tox because I kept running in some issues (#7108), so I hope that travis will be able to find if there are any problem left with the code!

[fabaff]

Thanks 🐦