deployment fails with "chmod: changing permissions of '/mnt/tmp': Operation not permitted"

During juju-deploy wordpress deployment exits with:

ERROR juju.worker.uniter.operation runhook.go:107 hook "install" failed: exit status 1

Exact last few lines are:

nit-wordpress-0[3965]: 2016-04-07 08:51:33 INFO unit.wordpress/0.install logger.go:40 + juju-log 'Making /mnt/tmp dir ...'
unit-wordpress-0[3965]: 2016-04-07 08:51:33 INFO unit.wordpress/0.juju-log server.go:268 Making /mnt/tmp dir ...
unit-wordpress-0[3965]: 2016-04-07 08:51:33 INFO unit.wordpress/0.install logger.go:40 + mkdir -p /mnt/tmp
unit-wordpress-0[3965]: 2016-04-07 08:51:33 INFO unit.wordpress/0.install logger.go:40 + chmod 1777 /mnt/tmp
unit-wordpress-0[3965]: 2016-04-07 08:51:33 INFO unit.wordpress/0.install logger.go:40 chmod: changing permissions of '/mnt/tmp': Operation not permitted
unit-wordpress-0[3965]: 2016-04-07 08:51:33 ERROR juju.worker.uniter.operation runhook.go:107 hook "install" failed: exit status 1

This is step right after installing knfsd. The step is defined in wordpress/hooks/install:

[...]
modprobe nfs || true

juju-log "Making /mnt/tmp dir ..."
mkdir -p /mnt/tmp
chmod 1777 /mnt/tmp
[...]

In this particular scenario, /mnt is mounted on /dev/vdb, which is a vfat partition:

cat /proc/mounts
[...]
dev/vdb /mnt vfat rw,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro 0 0
[...]

charm should be able to deal with this situation, possibly logging a warning, that /mnt is formatted as filesystem that does not support full POSIX access control semantics.

The environment is OpenStack. The deployment fails consequently on this step every time I run it.