xenapi: Move settings to their own config section

This commit changes xenapi_* config options to their own section
with the xenapi_ prefix removed where appropriate.  The change
is done in a backwards compatible fashion such that previous
nova.conf files still work.

As part of this update the sample file and text strings are
also updated to reflect new config options.  Test cases have
been adjusted to handle the config hierarchy.

2642897d6f5b8a211424519d99feddc89c3bf180 was followed to
implement this change.

Change-Id: I2e9f720ddce284fc112d6a5651fd277e6e31a17a
Closes-Bug: 1207253
DocImpact
Merge "xenapi: Move settings to their own config section"