Update mysql allowed_hosts conditional statement

In the origin ceilometer::db::mysql, if the value of $allowed_hosts
contains or equals to $host, then puppet will complain duplicate
declaration error. This patch is aim to update the allowed_hosts
conditonal statement in ceilometer::db::mysql.

There are two cases to pass $allowed_hosts to $real_allowed_hosts:

   - If $allowed_hosts is array,then remove $host from $allowed_hosts;
   - elsif $allowed_hosts is string and not equivalent to $host;

At last, if $real_allowed_hosts is not undef, then run
ceilometer::db::mysql::host_access

Fix bug 1206444

Change-Id: Ib6c33e6a27113d01326bee32c105fea4003059ce