roomservice: Use ElementTree.indent()

Replace custom indent() function with the built-in ElementTree.indent()
method, which was introduced in Python 3.9:
https://docs.python.org/3/library/xml.etree.elementtree.html#xml.etree.ElementTree.indent



Change-Id: I45c513a2f02a36c012490db9b05b8fa1eec356ca
Signed-off-by: Jyotiraditya Panda <jyotiraditya@aospa.co>