vtysh: Add an option to set banner motd from an input

vtysh: Add an option to set banner motd from an input

This allows to set motd from an input instead of creating a file.

Example:

root@exit2-debian-9:~/frr# telnet 127.0.0.1 2605
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
Hello, this is bgpd
User Access Verification

Password:
exit2-debian-9> enable
exit2-debian-9# sh run

Current configuration:
!
frr version 7.3-dev-MyOwnFRRVersion
frr defaults traditional
!
hostname exit2-debian-9
password belekas
log file /var/log/frr/labas.log
log syslog informational
banner motd line Hello, this is bgpd
!
!
!
line vty
!
end
exit2-debian-9#

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>