fix: force_no_enable enabled on ios and nx only

[tynany]

As per NAPALM documentation, force_no_enable is only used for Cisco NXOS and IOS devices, but is currently implemented for all NAPALM drivers that use _netmiko_open, which breaks those drivers as it is unlikely the force_no_enable method has been implemented.

This MR only enables force_no_enable for the Cisco devices types (cisco_ios_telnet, cisco_ios and cisco_nxos).

Below is an example error output of a NAPALM driver that uses _netmiko_open but does not implement the force_no_enable method:

'$insertNAPALMDriveName' object has no attribute 'force_no_enable'

[ktbyers]

Hmmm, how about we do a try/except and gracefully handle the AttributeError exception instead?

I am pretty strongly against doing a platform conditional check.

[tynany]

Good call -- try/except is a much better idea. I have pushed a new commit

[mirceaulinic]

LGTM, but Black seems unhappy about something, could you please check? https://travis-ci.org/github/napalm-automation/napalm/jobs/700735592

[mirceaulinic]

Bump @tynany

[coveralls]

Coverage remained the same at 0.0% when pulling 57a5997 on tynany:develop into 052ced8 on napalm-automation:develop.

[tynany]

Apologies for the delay in getting back to you. Travis is now happy -- https://travis-ci.org/github/napalm-automation/napalm/builds/706059614

[mirceaulinic]

Now that I'm looking at this again, it does look a little strange, wondering if the following would be more readable:

if not getattr(self, 'force_no_enable', False):
    self._netmiko_device.enable()

[mirceaulinic]

Any thoughts on this @tynany / @ktbyers ?

[ktbyers]

I think the try/except is fine.

[ktbyers]

This PR looks good to me so I am fine with merging it.