Fix undefined local variable error in draining_servers.rb

HBASE-21812 replaced a for-loop with an each block. Each block introduces a new scope, so a local variable defined inside it cannot be accessed afterwards.

  NameError: undefined local variable or method `admin' for main:Object
    getServerNames at /opt/khp/hbase/bin/draining_servers.rb:81
        addServers at /opt/khp/hbase/bin/draining_servers.rb:88
            <main> at /opt/khp/hbase/bin/draining_servers.rb:146

for i in [1, 2, 3]
  a = i
end
puts a
  # 3

[4, 5, 6].each do |i|
  b = i
end
puts b
  # undefined local variable or method `b'

We can define the admin local variable in the current scope beforehand, and we can still access it after the block.