tools: Add missing keyword `encapsulation` in frr-reload

[cscarpitta]

Consider the following scenario.

You start from the configuration below:

!
segment-routing
 srv6
  encapsulation
   source-address fc00:0:1::1
  !
 !
!

Then you change the source address:

r1# configure
r1(config)# segment-routing
r1(config-sr)# srv6
r1(config-srv6)# encapsulation
r1(config-srv6-encap) source-address 1::1

And finally, reload the configuration python3 frr-reload.py --reload /etc/frr/frr.conf

frr-reload returns the error below:

Failed to execute segment-routing  srv6   no source-address 1::1  exit exit
"segment-routing --  srv6 --   no source-address 1::1 --  exit -- exit" we failed to remove this command
% Unknown command:   no source-address 1::1

[79975|mgmtd] sending configuration
line 3: % Unknown command[76]:   source-address fc00:0:1::1
[79975|mgmtd] Configuration file[/etc/frr/frr.conf] processing failure: 2

The reason is that the keyword encapsulation is missing in frr-reload.

This patch adds the missing keyword encapsulation.