Expose Juniper format option for get_config so JSON format can be selected

[robobeaver6]

Description of Issue/Question

Feature Request:

For get_config against Juniper switches, currently the format option is hard coded to "format"="text". Juniper supports JSON format for the show configuration, which is much better to work with programmatically. The underlying jnpr.junos.Device module supports JSON format.

Did you follow the steps from https://github.com/napalm-automation/napalm#faq

(Place an x between the square brackets where applicable)

• Yes
• [] No

Setup

N/A

napalm version

(Paste verbatim output from pip freeze | grep napalm between quotes below)

napalm==2.5.0
napalm-asa==0.1.1

Network operating system version

(Paste verbatim output from show version - or equivalent - between quotes below)

Model: ex4300-48p
Junos: 18.1R3-S7.1

Steps to Reproduce the Issue

N/A

Error Traceback

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[mirceaulinic]

This sounds good to me. If you're willing to open a PR for this, please make sure to add the argument for all the core drivers (even though for some it may not make sense, the NAPALM API must be consistent).