Ability to read connection details from MySQL config file passed by --defaults-file flag

The following have not been implemented:

• --defaults-file

The absence of the ability to use a Mysql Config (my.cnf) file for connection config is bad for automation:

• it forces a password to be set in the command line
• it leads to the possibility that the password appears in logs and is shipped off somewhere
• it removes the ability to check if a password should change by detecting a change in the cnf file

Use case: 

As PMM user I need to be able to specify  --defaults-file flag as a path to .cnf file so that I can skip passing --user-name and --password  options while adding DB and they will be extracted from config file 

Acceptance criteria:

• User able to specify a path to defaults file
• If username and password is passed along with defaults path, the flags have highest priority and values (username and password) from file are ignored
• Parsing defaults file should be done by pmm-agent****

UI: 

pmm-admin add mysql --defaults-file=~/my.sandbox.cnf  MyDB 127.0.0.1 

 Also, Feature should be available by API (for future UI  improvements) and follow common logic - implement logic in pmm-agent, not  pmm-admin 

Note:

• this is username and password related. We are not adding support of other fields from config file
• If password changed  - user need to re-add Service to the system, as it goes with usual --password/–username  flags 

Out of scope:

• Handling encrypted credentials.

Suggested solution:

• Add --defaults-file flag to pmm-admin add mysql command
• Add new field `defaults_file` to management API for MySQL
• Add new action to extract credentials from defaults file
• Extract username and password on pmm-agent side
  • Use ini parser for it.
• Return Username and password from the action
• Save returned username and password to agent model
• Check connection

How to test:

1. Run PMM server.
2. Run MySQL.
3. Run PMM agent and prepare defaults file. e.g.:

[client] 
host=127.0.0.1 
port=12345 
user=root123 
password=root-password123  

Please remember to change credentials for proper one for MySQL database.
4. Use pmm-admin with --defaults-file parameter and path to file 

pmm-admin add mysql --defaults-file=~/.my.cnf 

5. You can replace username/password just to check if have highest priority:

pmm-admin add mysql --defaults-file=~/.my.cnf --username=bad 
pmm-admin add mysql --defaults-file=~/.my.cnf --password=wrong

6. It should throws credentials error during connection test