Reference DEFAULT_SERVICETYPE OptGoup names in lowercase

Reference DEFAULT_SERVICETYPE OptGoup names in lowercase

To have consistent option group name format across projects,
oslo.config now normalizes all non lowercase group names to lowercase
when loading conf files.
Thus conf files are backwards compatible but option group references
in the code must now use lowercase before we update oslo.config.

This patch replaces all 'DEFAULT_SERVICETYPE' references to
'default_servicetype'

Change-Id: Iccab055c4664e11c9618706e594416194065048b