in saltinit, avoid writing to user.conf if it exists

elirequested to merge
github/fork/waybeforenow/master into master

Context: I'm trying to mount a Kubernetes ConfigMap as a volume on /etc/salt/master.d inside the container. But this makes the directory unavoidably read-only due to kubernetes/kubernetes#62099, so I'm stuck with the following crash inside saltinit:

File "/usr/local/bin/saltinit", line 54, in main
    with open("/etc/salt/master.d/user.conf", "w") as userfile:
OSError: [Errno 30] Read-only file system: '/etc/salt/master.d/user.conf'

I can prevent the write to api.conf by providing my own definition for it inside the ConfigMap, and the write to master.conf by leaving SALT_MASTER_CONFIG unset. But there is no conditional guarding the write to user.conf, so I can't head off this particular crash (other than by configuring it as a minion or proxy, which is self-defeating). Do you think it's reasonable to add this conditional here?

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