BZ#1142182 - provisioned hosts have wrong timezone

https://bugzilla.redhat.com/show_bug.cgi?id=1142182

This adds support for setting timezone. The timezone will be set both
on the Foreman host and on the provisioned hosts. The timezone
defaults to the timezone currently set on the host where
staypuft-installer is run. The implementation should be forward
compatible for when Staypuft will run on EL 7 variants.

How i tested:

Set timezone to "Pacific/Auckland". On Staypuft host after running the installer:

    [root@r6-staypuft ~]# cat /etc/sysconfig/clock
    ZONE="Pacific/Auckland"
    [root@r6-staypuft ~]# date
    So zář 20 03:10:42 NZST 2014

On provisioned host:

    [root@r7-sp1 ~]# timedatectl 
          Local time: So 2014-09-20 03:11:57 NZST
      Universal time: Pá 2014-09-19 15:11:57 UTC
            RTC time: Pá 2014-09-19 15:11:57
            Timezone: Pacific/Auckland (NZST, +1200)
         NTP enabled: no
    NTP synchronized: no
     RTC in local TZ: no
          DST active: no
     Last DST change: DST ended at
                      Ne 2014-04-06 02:59:59 NZDT
                      Ne 2014-04-06 02:00:00 NZST
     Next DST change: DST begins (the clock jumps one hour forward) at
                      Ne 2014-09-28 01:59:59 NZST
                      Ne 2014-09-28 03:00:00 NZDT

better to rescue StandardError, we don't want to rescue from exceptions like NoMemoryError...