Add ssh public key support to the asuswrt component

[mtreinish]

Description: Add support for using an ssh public key to the asuswrt device tracker module

Related issue (if applicable): fixes #

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

Example entry for configuration.yaml (if applicable):

Checklist:

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

• Documentation added/updated in home-assistant.io

If code communicates with devices:

• 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.

The pexpect.pxssh module has support for using public key
authentication. [1] This commit adds support for leveraging that and
establishing a ssh connection with a public key instead of a password.

[1] http://pexpect.readthedocs.io/en/stable/api/pxssh.html#pexpect.pxssh.pxssh.login

[balloob]

Looks good. Few pep257 issues found, we can merge once those are fixed.

[mtreinish]

I updated the commit to include the missing docstrings

[balloob]

Looks good! 🐬