Support krb5.conf without includedir.

The version of krb5 available on el5 doesn't support the "includedir"
option in krb5.conf. This change alters the way that krb5.conf is
generated to no longer require the includedir option.

Additionally the management of krb5.conf has been moved into the
separate txwinrm.krb5 module to simplify the utils module.

Refs ZEN-9389.
Use separate credential cache for each user.

This is necessary because kinit destroys all cached credentials when
initializing a new credential.

Refs ZEN-9389.
Merge pull request #27 from zenoss/feature/ZEN-9389

Please review branch feature/ZEN-9389