For non-admin users are unable to login to InfluxDB

### Bug report

**System info:** [Include InfluxDB version, operating system name, and other relevant details]
Influx v0.12

**Steps to reproduce:**
1. Created a user called "influx_reader"
2. Among 10 DBs created, granted "ALL PRIVILEGES" to DB6, DB9 (to 2 databases)
3. When I tried login with influx_reader, I get the following error
Server returned error: error authorizing query: emmo not authorized to execute statement 'SHOW DATABASES', requires admin privilege

**Expected behavior:** [What you expected to happen]
User should be able to login. And should see the DB6, DB8 under databases dropdown at the top. The show databases command should return the DBs where the permissions are granted

**Actual behavior:** [What actually happened]
Server returned error: error authorizing query: emmo not authorized to execute statement 'SHOW DATABASES', requires admin privilege

**Additional info:** [Include gist of relevant config, logs, etc.]

There should be addition feature to create "read-only" user who can read all databases.

Also, if this is an issue of for performance, locking, etc the following commands are useful to create debug information for the team.

If I grant ALL to infux_reader, then the user will become admin user and is able to create new database, even though the "show grants for influx_reader" shows "ALL PRIVILEGES" access only to DB6 & DB9.

No files, tests, or code entry points are identified in the report. Begin by reproducing the InfluxDB v0.12 case with a non-admin user granted privileges on selected databases, then determine the intended behavior for login and SHOW DATABASES; done requires resolving the permission model without unintentionally granting administrator access.

Written by the indexing model from the issue text.