updates postfix hostname 'monitor.securedrop' -> 'ossec.server'
updates postfix hostname 'monitor.securedrop' -> 'ossec.server'

Postfix config was changed in 3fca1a5 in response to audit finding,
and now the spec test checks for the correct string.

mark-in/securedrop-prov-upstream,mark-in/securedrop-prov-upstream,mark-in/securedrop-prov-upstream,mark-in/securedrop-prov-upstream