out_custom_calyptia: enable machine_id config property.

[niedbalski]

if machine_id is set, that string value will be used
when registering the agent, if not given, a locally
generated one will be used.

Testing
Before we can approve your change; please submit the following in a comment:

• Example configuration file for the change

[CUSTOM]
name calyptia
api_key key-id
machine_id 0xdeadbeef
add_label pipeline_id moguri
add_label pipeline_name testing
log_level debug
calyptia_host localhost
calyptia_port 5000
calyptia_tls off
calyptia_tls.verify off

• [X ] Debug log output from testing the change

without specifying machine_id

[2021/10/20 14:16:07] [debug] [stdout:stdout.0] created event channels: read=20 write=21
[2021/10/20 14:16:07] [debug] [calyptia:calyptia.1] created event channels: read=22 write=23
[2021/10/20 14:16:07] [debug] [output:calyptia:calyptia.1] machine_id=2bda59b7000f87707d080ffd7e60f838657fb3bf8c3f065464662deca2e07927
[2021/10/20 14:16:07] [debug] [http_client] not using http_proxy for header
[2021/10/20 14:16:07] [ info] [output:calyptia:calyptia.1] connected to Calyptia, agent_id='1a75a8ea-093e-42cf-9315-aa6c78fed24d'
[2021/10/20 14:16:07] [debug] [upstream] KA connection #24 to localhost:5000 is now available

with specyfing machine_id

[2021/10/20 14:16:43] [debug] [output:calyptia:calyptia.1] machine_id=0xdeadbeef
[2021/10/20 14:16:43] [debug] [http_client] not using http_proxy for header
[2021/10/20 14:16:43] [ info] [output:calyptia:calyptia.1] connected to Calyptia, agent_id='4ba69642-df23-4202-86ae-5bea7b1bdd86'
[2021/10/20 14:16:43] [debug] [upstream] KA connection #24 to localhost:5000 is now available

• Attached Valgrind output that shows no leaks or memory corruption was found
  [2021/10/20 14:32:08] [ info] [engine] service stopped
  ==161677==
  ==161677== HEAP SUMMARY:
  ==161677== in use at exit: 0 bytes in 0 blocks
  ==161677== total heap usage: 1,935 allocs, 1,936 frees, 572,620 bytes allocated
  ==161677==
  ==161677== All heap blocks were freed -- no leaks are possible
  ==161677==
  ==161677== For lists of detected and suppressed errors, rerun with: -s
  ==161677== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)