IPaddr2: add support for network interfaces altname in the monitor operation

[waays-tl]

Currently, it's not possible to use an interface's altname by specifying it in the nic= proprerty because the monitor routine fails to recognize that the IP is configured on the proper interface.

This patch fixes that by returning all the altname properties of an interface alongside its main name.

[knet-jenkins]

Can one of the admins check and authorise this run please: https://ci.kronosnet.org/job/resource-agents/job/resource-agents-pipeline/job/PR-2027/1/input

[oalbrigt]

We should only return one interface name here, so you could do an if/else like if [ -n "$iface_altname" ]; then.

[waays-tl]

There's a comment a few lines up that says:
# Its output are interface names devided by spaces (e.g., "eth0 eth1").

According to my tests, it works when returning both names (added set -x):

root! server:~# pcs resource debug-monitor vipAB
Operation monitor for vipAB (ocf:heartbeat:IPaddr2) returned: 'ok' (0)
 >  stderr: + local ipaddr=192.xxxxxxx
 >  stderr: + local netmask=27
 >  stderr: + ip -o -f inet addr show
 >  stderr: + grep \ 192.xxxxxxxxx/27
 >  stderr: + cut -d   -f2
 >  stderr: + grep -v ^ipsec[0-9][0-9]*$
 >  stderr: + local iface=enp8s0f1
 >  stderr: + ip -f inet link show dev enp8s0f1
 >  stderr: + grep altname
 >  stderr: + sed s/^[[:space:]]*//; s/altname //g
 >  stderr: + tr \n  
 >  stderr: + local iface_altname=nic-third 
 >  stderr: + echo enp8s0f1 nic-third 
 >  stderr: + return 0