'pmm-admin status --wait' option added to allow for configurable delay in checking status of pmm-agent

For better integration with k8s, there should be a way to wait for pmm-agent to become fully configured and connected.

Add -wait flag to pmm-admin status command. Flag's default value(0s) should not change the command behavior. Otherwise, the flag's value should contain duration (like in pmm-admin status -wait=30s). If given, pmm-admin should wait for pmm-agent to become connected for that maximum time. It should retry checking pmm-agent's status with a one-second delay after the previous request failed or returned information about pmm-agent not being connected. It should retry silently unless -debug or -trace is given. After pmm-agent is connected or that time elapsed, it should behave as it is now (i. e. produce normal output or exit with an appropriate error message and non-zero status code).

That flag should cover the following cases:

• when pmm-agent is not running (what currently produces "Failed to get PMM Agent status from local pmm-agent: Post "http://127.0.0.1:7777/local/Status": dial tcp 127.0.0.1:7777: connect: connection refused." error);
• when pmm-agent is running but not configured (what currently produces "Failed to get PMM Agent status from local pmm-agent: pmm-agent is running, but not set up." error);
• when pmm-agent is running and configured, but not connected to PMM Server (what currently produces "Failed to get PMM Agent status from local pmm-agent: pmm-agent is not connected to PMM Server." too, but actually a different case).

How to test:

• Setup PMM Server
• Run pmm-admin status --wait=30s
  • After 30s should return error "Failed to get PMM Agent status from local pmm-agent: pmm-agent is running, but not set up."
• Configure pmm-agent
• Run pmm-admin status --wait=30s
  • Should return status immediately
• Stop PMM Server
• Run pmm-admin status --wait=30s
  • After 30s should return error "Failed to get PMM Agent status from local pmm-agent: pmm-agent is not connected to PMM Server."
• Run pmm-admin status --wait=30s and start PMM Server
  • After a few seconds status should be returned
• Stop pmm-agent
• Run pmm-admin status --wait=30s
  • After 30s should return error "Failed to get PMM Agent status from local pmm-agent: Post "http://127.0.0.1:7777/local/Status": dial tcp 127.0.0.1:7777: connect: connection refused."
• Run pmm-admin status --wait=30s and start pmm-agent
  • After a few seconds status should be returned

Documentation:

Documentation should be added to pmm-admin man page.

--wait values expected in go format `30s`, `5m`, `1h`