s3api: Make allowable clock skew configurable

While we're at it, make the default match AWS's 15 minute limit (instead
of our old 5 minute limit).

UpgradeImpact
=============
This (somewhat) weakens some security protections for requests over the
S3 API; operators may want to preserve the prior behavior by setting

    allowable_clock_skew = 300

in the [filter:s3api] section of their proxy-server.conf

Co-Authored-By: Alistair Coles <alistairncoles@gmail.com>
Change-Id: I0da777fcccf056e537b48af4d3277835b265d5c9

etc/proxy-server.conf-sample

@@ -613,6 +613,10 @@ use = egg:swift#s3api
 # AWS S3 document says that each part must be at least 5 MB in a multipart
 # upload, except the last part.
 # min_segment_size = 5242880
+#
+# AWS allows clock skew up to 15 mins; note that older versions of swift/swift3
+# allowed at most 5 mins.
+# allowable_clock_skew = 900
 
 # You can override the default log routing for this filter here:
 # log_name = s3api