re-use install instead of update function

Previous commit:http://gerrit.ovirt.org/#/c/25470/
to add --update into edit-node to update packages.but this patch
didn't achieve "_run_yum_update" functions
Now add "_run_yum_update" function into edit-node tools to make
this --update options really could work.`
Also add this --update options into edit-node man page.

Change-Id: I778d4255e6b0f3d76e576675356e8e11263ed935
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1073720
Signed-off-by: hadong <hadong@redhat.com>