Fix Kerberos keytab string handling

Assuming a configuration where FreeNfiAS is configured to use AD, using an imported keytab file, where the NetBIOS style SPN is used. (see attached image)

The NetBIOS style hostname usually includes a '$' sign at the end.

I.E nd-bsd-1$@REALM

observed behaviour:

Upon (re)start of the AD Directory Service on FreeNAS, cachetool passes the SPN 'nd-bsd-1REALM' string to kinit, which inevitably fails. (Mind the missing '$@' signs)

This produces the following log messages:

Sep  6 03:30:21 nd-bsd-1 /cachetool.py: [common.pipesubr:65] Popen()ing: /usr/bin/kinit --renewable -k -t /etc/kerberos/nd-bsd-1 ND-BSD-1LAN.DOMAIN.AT
Sep  6 03:30:25 nd-bsd-1 /cachetool.py: [common.pipesubr:65] Popen()ing: /usr/bin/kinit --renewable -k -t /etc/kerberos/nd-bsd-1 ND-BSD-1LAN.DOMAIN.AT
Sep  6 10:34:16 nd-bsd-1 uwsgi: [common.pipesubr:65] Popen()ing: /usr/bin/kinit --renewable -k -t /etc/kerberos/nd-bsd-1 ND-BSD-1LAN.DOMAIN.AT

Whereas the kerberos_start script properly calls

Sep  6 10:34:47 nd-bsd-1 ActiveDirectory: kerberos_start: /usr/bin/kinit --renewable -t /etc/kerberos/nd-bsd-1 -k ND-BSD-1$@LAN.DOMAIN.AT

This is most likely, due to improper escaping of the SPN string passed on to kinit, causing '$' to be interpreted as a special character.

expected behaviour:

The string passed on to kinit by cachetool.py (?) is being properly escaped.