Add source_interface argument to ping

[chadell]

Some vendors allow the definition of a source interface instead of the source address for the ping command with different keyword. For instance, Junos.
Others don't use different keyword, so both arguments (source and source_interface) will end up using the same one.

[mirceaulinic]

Looks good. Any chance you could add the argument for the remaining platforms? (Even without actually implementing, just logging a warning when the argument is being used it would be just fine - see for example like this: https://github.com/napalm-automation/napalm/blob/develop/napalm/junos/junos.py#L1548-L1550)

[chadell]

Looks good. Any chance you could add the argument for the remaining platforms? (Even without actually implementing, just logging a warning when the argument is being used it would be just fine - see for example like this: https://github.com/napalm-automation/napalm/blob/develop/napalm/junos/junos.py#L1548-L1550)

When you say "other platforms" which ones are you thinking on?
Maybe I have skipped some but I tried to add support to all the ping methods in the repository, where junos and eos have a differentiated argument for the source_interface case and for ios and nxos it overloads the already available source argument as it can take or a source_ipaddress or source_interface.

[mirceaulinic]

You're right @chadell, I assumed the XR drivers implement ping too... Thanks!