Setting data retention via env on docker run prevents changes in PMM settings

Impact on the user:

• User is unable to change Settings if some other parameters/settings passed over Env. Variables 

Steps to reproduce:

1.  start PMM 

docker run -d -p 80:80 -p 443:443 --volumes-from pmm-puneet-20200417050711-12155-data --name=pmm-puneet-20200417050711-12155-server -e DATA_RETENTION=48h -e DISABLE_TELEMETRY=true perconalab/pmm-server:dev-latest
Go to  PMM Settings page and change any options, like  Custom resolution  or enable any Feature Toggle 

Actual result:

• Error because one of the parameters can't be changed, and other parameters you  expected not saved 

Expected Result:

• No changes in parameters specified by Env. variables if they are not changed by the user 
• no option for the user to change parameters passed by Env. variable  
• Parameters are not passed by Env. variables and changed - changed successfully  
   

Workaround:
Use API calls to change only "not disabled" (not passed by env. variable) parameters 

Original report:
 

Build a PMM2 monitor using the docker image.

Set 4392h (183 days) of retention using the -e DATA_RETENTION variable in the docker run command

Attempt to enable the security threat tool generates the error: "Data retention for queries is set via DATA_RETENTION environment variable" and STT is not enabled.

Data retention shown in the UI accurately reflects that of the set variable and is not being changed.