Simplify keytab creation in kadmin and RPC tests

Simplify keytab creation in kadmin and RPC tests

In init_db and init.exp, do not create an ovsec_adm.keytab; kadmind
has authenticated directly against the KDB since commit
416d9a774090ee78c30a844025887bd2b9e79d16.  Since we no longer create
ovsec_adkm principals, perform the deletion and recreation tests with
kadmin/ principals.

In helpers.exp, use kadmin to create the server keytab file, instead
of using make-host-keytab.pl.

Remove environment variable settings for make-host-keytab.pl from
scripts that no longer use it.