Fix exception when api_extensions is set to empty

test.is_extension_enabled() is not checking if
config_dict[service] list is empty

Added a check if config_dict[service] is empty the
function will return False which means no extensions
are enabled

Change-Id: I2ee64f205c393637b5fc65897f1489292781c0be
Closes-Bug: 1342516
Add links to the field guide index

This commit takes the subsection summary in the field guid index
and adds internal links to the field guide page. Previously when
viewing the field guide docs it wasn't clear how to navigate between
all the different test class pages, this is remedied by adding a link.

Change-Id: Ica93a92c57e5d5d805a8b25d7c7e17509695152b
Add unit test section to the field guide index

The unit tests are another class of tests in tempest, but they were
excluded from the field guide index. This commit adds a section at the
bottom of the index for the unit test field guide.

Change-Id: I0f081c7a94bca46322381f8a5d28d7cadf334bbc
Merge "Add links to the field guide index"
Merge "Add unit test section to the field guide index"
Merge "Fix exception when api_extensions is set to empty"