As part of the profile environment only the kiwi_install_volid
variable existed with the default value 'INSTALL'. This information
is correct when building an oem image with installiso set to
true. The resulting .install.iso image has the configured volid
or the mentioned default 'INSTALL'. However, when building a live
ISO image the default volid value is set to 'CDROM' but the profile
environment contains kiwi_install_volid which is misleading and
in case no volid is provided in the configuration file, the
default value would also be a wrong information. This commit cleans
up the mess and provides kiwi_install_volid for install ISO media
and kiwi_live_volid for Live ISO media with their respective
default values
