Implement install_os method for juniper

[lvrfrc87]

Missing method prevent OS Upgrades app to support Juniper platform

pyntc/pyntc/devices/jnpr_device.py

Line 334 in 848377c

def install_os(self, image_name, **vendor_specifics):

Issue reference in OS Upgrades repo --> https://github.com/networktocode-llc/nautobot-app-os-upgrades/issues/123