frontend:  node upgrade detection improvement

In commit 5233548 the comparison failed because revision strings
from the node's OS might contain extra digits in the date like 20130212.0.

This patch parses the string from vds.getHostOs() as shown below
and splits it into major, minor, build and revision. However, in the revision
field it compares each segment delimited by dots and makes a comparison
between the node's OS and the ISO until it reaches a letter,
like "fc18" or "el6", which is derived from the rpm.

oVirt Node - 2.6.0 - 20130212.0.fc18
oVirt Node - 2.6.0 - 20130212.fc18
oVirt Node - 3.0.0 - 5.0.5.vdsm.fc19
RHEV Hypervisor - 6.4 - 20130709.0.el6_4
RHEV Hypervisor - 6.4 - 20130709.0.el6_4

Change-Id: I76bd2948524110ec80923e855bd1f49e09e17046
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>