ipaserver: Allow configuration of test server name.

As the server name was hard coded, when running tests for ipasever
module using a server not name as 'ipaserver', the tests would fail.

This patch allows the configuration of the server name using the
variable 'ipa_server_name', and if not provided, try to infer the name
from the ansible_hostname variable, or, if not possible, defaults to
'ipaserver'.

The same behavior is applied to 'ipasever_domain', with the default
being 'ipa.test'.