ipaclient: Do not fail on rmkeytab error #7

Due to commit f3f9672d527008dc741ac90aa465bac842eea08d (ipa-rmkeytab: Check
return value of krb5_kt_(start|end)_seq_get) in IPA 4.9.2 there is a new
error reported for ipa-rmkeytab in case of a non existing keytab file.
Using ipa-rmkeytab now results in the error #7 in this case.

The client role is using ipa-rmkeytab and needs to ignore error #7 also.

Fixes: #510 (ipa-client installation with OTP is failed with error code 7
             (keytab: /usr/sbin/ipa-rmkeytab returned 7))