Add config syntax support for 'transport=telnet'

[The-Loeki]

I just ran into this issue & hacked up this PR :)

Fixes https://github.com/napalm-automation/napalm-ios/issues/132

[coveralls]

Coverage decreased (-0.2%) to 70.129% when pulling 3224f96be7181d092f177610e6879c84e6b8a0c6 on The-Loeki:allow-telnet into b69c73b7cc9620567006c2e36212d65044abeb68 on napalm-automation:develop.

[coveralls]

Coverage increased (+4.5%) to 74.785% when pulling 3224f96be7181d092f177610e6879c84e6b8a0c6 on The-Loeki:allow-telnet into b69c73b7cc9620567006c2e36212d65044abeb68 on napalm-automation:develop.

[dbarrosop]

Could you also document this optional_arg here, please?

https://github.com/napalm-automation/napalm/blob/master/docs/support/index.rst#list-of-supported-optional-arguments

Something like:

transport (ios, eos) - Transport to use when connecting to the device.  Supported transports are the ones supported by the underlying library. Defaults to https on EOS and ssh on IOS.

Thanks! :)

[mirceaulinic]

Postponed this for 0.8.0 as per @ktbyers' suggestion

[matejv]

is_alive method needs updating as well.

When using telnet remote_conn will be a telnetlib.Telnet instance that doesn't have a transport attribute. I'm not sure what is the right way to check if a telnet connection is active.

[dbarrosop]

Note there are CI issues we have to solve before merging.

[ktbyers]

I will update this. Hopefully this week or next week.

[austind]

@ktbyers Let me know if you need any help testing the PR

[ktbyers]

This change is incorporated here:

https://github.com/napalm-automation/napalm-ios/pull/192

[austind]

Successfully ran get_facts() against two telnet devices. Thank you!

[ktbyers]

@austind Nice!