Add support for installation of `bench` on linux mint

Issue: Easy Install
Running the install script on linux mint fails

**Do the checklist before filing an issue:**
- [x] Did you retain the logfile (path of logfile is shared while the script is run)? We definitely **need** the logfile to debug any easy install related issues.
- [x] Is this something you can debug and fix? Send a pull request! Bug fixes and documentation fixes are welcome

**Distro Information (Required)**

1. uname -a
```Linux gunic0rn 5.8.0-45-generic frappe/bench#51~20.04.1-Ubuntu SMP Tue Feb 23 13:46:31 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux```

2. cat /etc/*-release

```
DISTRIB_ID=LinuxMint
DISTRIB_RELEASE=20.1
DISTRIB_CODENAME=ulyssa
DISTRIB_DESCRIPTION="Linux Mint 20.1 Ulyssa"
NAME="Linux Mint"
VERSION="20.1 (Ulyssa)"
ID=linuxmint
ID_LIKE=ubuntu
PRETTY_NAME="Linux Mint 20.1"
VERSION_ID="20.1"
HOME_URL="https://www.linuxmint.com/"
SUPPORT_URL="https://forums.linuxmint.com/"
BUG_REPORT_URL="http://linuxmint-troubleshooting-guide.readthedocs.io/en/latest/"
PRIVACY_POLICY_URL="https://www.linuxmint.com/"
VERSION_CODENAME=ulyssa
UBUNTU_CODENAME=focal
```

**Command Run (Required)**

```bash
sudo python3 install.py --production
```

**Log File (Required)**

```bash
Logs are saved under /tmp/logs/easy-install__2021-03-18__16-20.log
curl already installed!
wget already installed!
git already installed!
pip3 already installed!
ERROR: launchpadlib 1.10.13 requires testresources, which is not installed.
pip3 already installed!
Checking System Compatibility...
Sorry, the installer doesn't support linux mint. Aborting installation!
```

**Screenshots**


**Additional context**

**Possible Solution**

This is a simple issue, that can be solved by adding linux mint to the list of supported environments and distributions.
```'linux mint': [19, 20]```

Start in install.py at the system compatibility check that rejects Linux Mint. Review how supported environments are represented, add Linux Mint 19 and 20 if consistent with the existing structure, and run the production install command on a supported test environment or verify the compatibility path no longer aborts for the reported distribution.

Written by the indexing model from the issue text.