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
Merge "Reference DEFAULT_SERVICETYPE OptGoup names in lowercase"