Save PKI token in a file for PXE deploy ramdisk

Vendor passthru API should be used for PXE deploy from ramdisk,
but we must have a valid PKI token for authorization. PKI token
saved in a file in the tftp root for further read from the ramdisk.

Change-Id: I8e5f1cf0a0c48874b694f24226613a8db4d49cd1
Port StringType class from WSME trunk

Port the StringType class and its tests from WSME trunk. The StringType
is needed for the implementation of our PatchType class that will make
use of the complex type validation of WSME to remove some of the custom
validation code.

See: https://review.openstack.org/#/c/60194/

Change-Id: If387ed915e4d0102112f93ed16eee13775b54f4c
Updated from global requirements

Change-Id: I5ee5c994a5e1feaadc92c129434292ae1954116b
Add -U to pip install command in tox.ini

Ask pip to upgrade packages in the virtualenv so that a long-lived
virtualenv is still kept up to date as external dependencies change.

Change-Id: Ife76a83be95fed7cc4b99bb50a97277d0751e76c
some fixes