tools: fix peer-group deletion in frr-reload

tools: fix peer-group deletion in frr-reload

All of peers and respective configs are wiped out when
pee-group is removed.

In an attempt to remove peer-group and its associated peers
configs via frr-reload fails if the peer-group is removed first.

To pass the peer-group config removal via frr-reload following
steps are taken:
Find the bgp context to which peer-group belongs.
Find the peer-group associated peer(s) and store them in a list.
Remove the peers config lines from the pending list.
Move the peer-group deletion line to end of the pending list so
any remaining peer-group associated config can be removed successfully.

The above steps take 3 iterations over the pending list and scales
linearly.

Ticket:2656351
Reviewed By:CCR-11575
Testing Done:

Broken:

config:
router bgp 5544
 neighbor PG1 peer-group
 neighbor PG1 remote-as external
 neighbor swp10 interface peer-group PG1
 neighbor swp10 timers 3 9

failed frr-reload log:
2021-05-17 22:02:42,608  INFO: Executed "router bgp 5544  no neighbor
PG1 peer-group"
2021-05-17 22:02:42,708  INFO: Failed to execute router bgp 5544  no
neighbor PG1 remote-as external
2021-05-17 22:02:42,808  INFO: Failed to execute router bgp 5544  no
neighbor PG1 remote-as
2021-05-17 22:02:42,906  INFO: Failed to execute router bgp 5544  no
neighbor PG1
2021-05-17 22:02:43,007  INFO: Failed to execute router bgp 5544  no
neighbor
2021-05-17 22:02:43,106  INFO: Failed to execute router bgp 5544  no
2021-05-17 22:02:43,106 ERROR: "router bgp 5544 --  no" we failed to
remove this command
2021-05-17 22:02:43,107 ERROR: % Create the peer-group or interface
first

With fix:
2021-05-17 22:05:27,687  INFO: Executed "router bgp 5544  no neighbor
PG1 remote-as external"
2021-05-17 22:05:27,791  INFO: Executed "router bgp 5544  no neighbor
PG1 peer-group"

Signed-off-by: Chirag Shah <chirag@nvidia.com>