In AnnexRepo.__init__(), we access .config twice for read-only
inspection and will access it once more in the next commit, so store
it as a local variable.

git-annex has an annex.retry option that instructs it to retry on
transfer failure, but it defaults to 0.  Add a datalad.annex.retry
option that defaults to 3 and is used as the value for annex.retry in
our calls to git-annex, taking care to not override annex.retry if the
user has configured it.

This implements @yarikoptic's proposal from dataladgh-2896 and will hopefully
help resolve some flaky test failures (e.g., dataladgh-3653 and dataladgh-4371).

Closes datalad#2896.