Introduce set_hostname option

This allows you to disable the modification of the hostname if required.
E.g. if you want to keep the DHCP provided hostname.

set_hostname is by default set to true.
Introduce set_hostname conditional flag for dtagcloud models
Introduce mirror options to configure the installation source

This allows to specificy a installation repository for preseed based distros
beside the default mirror location.

This option should be reused also for other distros/models.
Introduce mirror option for dtagcloud models
Introduce make_user option for preseed based models

This allows to control if default user should get created during
Razor provisioning.

Note: the default behavior for Ubuntu behavior changed to False
by default.

The same hardcoded user credentials are used in case make_user get
set to "true". Those also got now used for Debian Wheezy if make_user
is set to "true".
Enable make_user for dtagcloud models
Introduce boot_kernel_args and install_kernel_args

* install_kernel_args: controls kernel parameters of the distro installer
* boot_kernel_args: controls kernel parameters of the targted provisioned system

Later one go so for only implemented for preseed based models.
Introduce install_kernel_args and boot_kernel_args for dtagcloud models
Incooperated initial RAID configuration for compute nodes

based on Adrians proposal