Fixed variable comparisons in shutdown.sh and ready.sh

[erste]

Fixes #363 .

Changes proposed on the PR:

• You are using /bin/sh to run shutdown.sh and ready.sh, but sh in Debian containers don't understand "==" and "[[ ]]" operands.

[ese]

Thanks @erste have you tested it with alpine and debian?

[erste]

Yes. With auth and without it.

For Debian:

I have no name!@rfr-redisfailover6-0:/data$ sh -x /redis-readiness/ready.sh
+ ROLE=role
+ ROLE_MASTER=role:master
+ ROLE_SLAVE=role:slave
+ IN_SYNC=master_sync_in_progress:1
+ NO_MASTER=master_host:127.0.0.1
+ cmd=redis-cli
+ [ ! -z  ]
+ cmd=redis-cli info replication
+ echo redis-cli info replication | grep role | tr -d "\r" | tr -d "\n"
+ xargs -0 sh -c
+ role=role:master
+ check_master
+ exit 0

For alpine:

/data $ sh -x /redis-readiness/ready.sh
+ ROLE=role
+ ROLE_MASTER=role:master
+ ROLE_SLAVE=role:slave
+ IN_SYNC=master_sync_in_progress:1
+ NO_MASTER=master_host:127.0.0.1
+ cmd=redis-cli
+ '[' '!' -z  ]
+ cmd='redis-cli info replication'
+ echo 'redis-cli info replication | grep role | tr -d "\r" | tr -d "\n"'
+ xargs -0 sh -c
+ role=role:master
+ check_master
+ exit 0

[ese]

Perfect, thank you so much