updated 11/2/2014
Hi Arrey,

Note, be prepared for a filesystem relabeling for selinux after mounting the /mnt/sysimage using the rescue disk.

Note: I too noticed that unlike rhel6, the rhel7 rescue mode has no 'start networking' feature that is part of the rescue boot. If there is something there for that, I didn't find it immediately either. However, I was able to get the networking to work while in the rescue mode using the below steps in my case.

CAUTION: You production admins you... If you use linux rescue from a dvd/iso file, be excessively careful with SAN attached storage!!! If you press the wrong key and do not take your time, you could wipe out san-presented storage. You've been warned. I have had to rescue a system that presented SAN storage and had to tell the system "no" some 65 or so times NOT to wipe numerous LUNs. BE CAREFUL

Here's what I did...

• I booted a virtual system today into rescue mode using a RHEL 7 iso (a virtual dvd). Once I did that, I did the following.

NOTE: Do not use "mount -a" if the /etc/fstab contains mounts from an external source, since networking has not yet been started.
-- Note, After running the chroot command below, check if your /etc/sysconfig/network-scripts/ shows your networking files you created previously -- after doing the chroot command below.

# chroot /mnt/sysimage
# echo "now your ifcfg-ethX files ought to be visible, otherwise look into your /etc/fstab to mount your / file system"
# ls -l /etc/sysconfig/network-scripts/ifcfg*
# echo "see caution above about the mount command"
# mount -a
# df -PhT

• last command above showed everything mounted as anticipated

• Start the service ('service network start' works and systemctl start network.service fails (thank you Arrey Ashu)).
  I am curious and researching why 'systemctl start network.service fails in rescue mode

# service network start 
# echo "systemctl start network fails in rescue mode"

• My rhel7 system has networking previously established before I booted from the dvd, so networking is already configured
• My /etc/resolv.conf is configured properly to access a dns server on my network

# ping server55
64 bytes from server55.test.net (172.16.0.55): icmp_seq=1 ttl=241 time=87.0 ms
64 bytes from server55.test.net (172.16.0.55): icmp_seq=2 ttl=241 time=84.5 ms
64 bytes from server55.test.net (172.16.0.55): icmp_seq=3 ttl=241 time=87.3 ms

So I was able to 1) boot from a dvd (in the form of an iso file) and 2) I could start networking after mounting my file systems and using systemctl start network.service

Hope this helps...
Kind Regards

p.s. Use this method to reset a password on a RHEL 7 system from the Grub 2 boot menu. This link goes to Red Hat documentation. I add this for those who are able to boot from grub and are not forced to use a DVD. NOTE: Virtualized systems should take note of the first comment by the Red Hatter "Matthew Casperson" at this Red Hat solution 918283, and you may have to add "console=tty0" at the end of the line that begins with "linux16" if your output gets redirected to a serial terminal.