Stop hard-coding the Firefox Nightly version information in our configuration

[gabrielBusta]

This issue pertains to the merge duty step outlined here.

Currently, the Nightly version information is hard-coded into the shipit configuration file:

shipit/api/src/shipit_api/common/config.py

Lines 48 to 55 in 01cc5f5

	# FIREFOX_NIGHTLY version is hard coded and requires a human to update it after
	# the latest Nightly builds are available on CDNs after version bump (merge
	# day).
	# We could have used the in-tree version, but there can be race conditions,
	# e.g. version bumped, but still no builds available.
	#
	# This version also defines the mobile nightly version (i.e.: Fenix)
	FIREFOX_NIGHTLY = "127.0a1"

This requires a manual source change every time there is a new Nightly version, followed by a shipit deployment. To reduce manual intervention from release engineering, we should enhance shipit to make it possible to update the current Nightly version without needing code modifications or application deployments.

This improvement will help automate this merge-day task and supports the objectives of Issue #1418.

[gabrielBusta]

 # We could have used the in-tree version, but there can be race conditions, 
 # e.g. version bumped, but still no builds available

We could have an API to bump the version, if needed, from a shipitscript task in the Nightly graph. Kinda like bouncerscript:

current_version = get_nightly_version(product_name, path)
if current_version == payload_version:
    log.info("No-op. Nightly version is the same")
    continue

This task could be downstream of the builds somewhere to avoid these race conditions.