Support Public key authentication to connect to switches

[naved001]

This adds support for the Dell Powerconnect switch and Cisco Nexus.

Configuring the Dell N3000 to use pubkey was a mess. I did configure it and could login using my key, it wasn't smooth though.

naved:~/pubkey-ssh/hil$ ssh switch.ipmi.cluster -l naved

R5-PA-C01-U39# (I PRESSED any key here)
Password:Login of naved failed (then I pressed enter)
R5-PA-C01-U39>
R5-PA-C01-U39>en

R5-PA-C01-U39#   

Don't know why it tells me about login failed when I did login correctly? Ideally, I would want to reset the switch config and start with a clean slate, but I can't do it since it's a shared switch.

[zenhack]

@naved001, now that #923 is merged, rebasing should fix the tests.

[naved001]

I have refactored the n3000 too, but I couldn't test pubkey auth on it for the reasons described in the title. It works normally with password though.

For the other 2 switches, I ran *network deployment tests with both kind of users (password, and pub key).

[naved001]

Re: the prompt stuff in the dell switch.

If we send it some random string.

(Pdb) p console.after
'\r\n\r% Unrecognized command\r\nconsole#'

our string of interest is separated by new line so we can extract it.

But if we send a new line to the switch, this is how console.after looks like

(Pdb) p console.after
'\r\n\r\r\x1b[Kconsole#'

we can't get the prompt name from this since our string of interest in not separated by new lines. It has \x1b and [K in between, which seem like escape sequences for something (text formatting?).

Edit: apparently \x1b[ is some ANSI stuff (erase line or something)

[naved001]

@zenhack I think I should remove the fixme from dell.py because to me it doesn't look like we can do anything about it. I'll just leave the comment there explaining what's going on.

[naved001]

@zenhack updated the comments and docstring.

@mosayyebzadeh could you give this a second review?

[mosayyebzadeh]

Everything seems fine to me. Just change the document and I will approve it.

[naved001]

Just change the document and I will approve it.

@mosayyebzadeh what do you want changed in the documents?

[mosayyebzadeh]

Highlight this part in the document or make it noticeable: "There should be no "enable" password."

[naved001]

@mosayyebzadeh done. Also, I noticed that it was in the wrong section; so I put it up on top because it's applicable for all switches.

[zenhack]

LGTM, merging.