Exchange ipaclient_allow_repair and ipaclient_otp in Special Variables

The docuemntation of ipaclient_otp was not part of the pull request
102 (commit d1af0ff4). The role README has been updated.

Updated requirements for python3-gssapi

The description of ipaclient_on_master in the ipaclient README was
providing the information that the flag is also used for relicas. This
is not correct and has been removed.

Add Information about needed /usr/bin/kinit on the controller when OTP is used

This will result in a better role documentation on galaxy.

krb5 DNS discovery was not possible in cluster environments as the server
list from groups.ipaserver was used all the time. DNS discovery is though
only used if no servers are given.

The new setting ipaclient_no_dns_lookup has been added to make sure that
DNS lookup is used in the first place and can be disabled easily with this
setting. There is also a new way to override servers per client in the
inventory file with ipaclient_servers.

Two new settings have been added:

ipaclient_no_dns_lookup (bool, default: no)
  Set to 'yes' to use groups.ipaserver in cluster environments as servers
  for the clients. This deactivates DNS lookup in krb5.

ipaclient_servers (list of strings, default: undefined)
  Manually override list of servers for example in a cluster environment on
  a per client basis. The list of servers is normally taken from from
  groups.ipaserver in cluster environments.

This has been done to make clear that these are admin settings and to make
these settings consistent to ipaadmin_keytab.

The use od ipaclient_keytab for ipahost is not correct as the admin keytab
needs to be used here.