Polish new contributor welcome message hook script

* jeepyb/cmd/welcome_message.py: Several fixes... minor comment
corrections; use the commit gitsha instead of the Change-Id string
when submitting messages since the latter is not valid; close the
SSH session when we're done with it so Gerrit doesn't kill the
dropped socket; log stdout and stderr from SSH at appropriate
levels; pass the SSH user and key file as options instead of
positional arguments since we have limited control over the order of
hook parameters passed from Gerrit; match on the patchset as a str
instead of an int since it is not of the latter type.

Change-Id: Ib1661b16f6a82f8fd917ec273bfac67145cd5116