test_server_metadata.py - BP add-xml-support

This change partially implements blueprint add-xml-support

Addition of XML support to test_server_metadata.py
Modified servers_client.py to support the following
calls to check for xml response to server metadata API:
list_server_metadata,
set_server_metadata,
update_server_metadata,
get_server_metadata_item,
set_server_metadata_item,
delete_server_metadata_item.

Implement: Blueprint add-xml-support
Change-Id: If88b769022fefc80a933b76057a0481d4bc3fac6
Merge "test_server_metadata.py - BP add-xml-support"