Use fsfreeze instead of xfs_freeze

Summary:
fsfreeze is a standard part of modern linux distributions, and works
on xfs and other filesystems too.  So prefer it.  Also xfs_freeze was
giving an ENOTSUPPORTED error for me, while fsfreeze works fine.

I also moved the freezing into ec2-create-rolling-snapshot, rather
than the wrapper script, so that only the snapshotting command would
require freezing, not the whole script (which also deletes old
snapshots, etc).  This should keep the fs more responsive during
backups.  Also, I trust try/finally more than I trust sh's 'trap', so
I'm confident that the fs will always be unfrozen.

I support this with a new --freezedir option.

Use call() instead of Popen().  I missed that this function exists.

Also call split('\t') rather than just split, in case the volume name
has spaces in it or something.

Test Plan:
I copied this script manually over to toby and ran it, and it
successfully created a snapshot.  I still haven't tested any of the
deletion stuff; since the crontab is now installed, it should test
itself in about two weeks.

Reviewers: benkomalo, alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D2010