The parsing of tenant_name value from tempest.conf ends with an error as
the ConfigParser.get() function fails if a requested value is missing
in a config file.
This patch fixes the issue by using the fallback option of the
ConfigParser.get() function. Also, the unittest that checks the reading
of [identity] section from tempest.conf is fixed.
Change-Id: I9b3224f4ab8900a4954814844ef8c13e099ccae9