[BUG] file.keyvalue doesn't preserve indentation

Description
file.keyvalue doesn't preserve indentation of key:value pairs in configuration file.

Setup

1. Create the configuration file to be modified on the minion:

# cat << EOF > /tmp/test.txt
<VirtualHost *:80>
  ServerName YOUR_SERVER_FQDN
  ServerSignature Off

  RewriteEngine on
  RewriteCond %{HTTPS} !=on
  RewriteRule .* https://%{SERVER_NAME}%{REQUEST_URI} [NE,R,L]
</VirtualHost>
EOF

2. Create the following state:

file_keyvalue_test:
  file.keyvalue:
    - name: /tmp/test.txt
    - key: ServerName
    - value: host.example.com
    - separator: ' '
    - key_ignore_case: False
    - append_if_not_found: False

Steps to Reproduce the behavior
3. Apply the state:

# salt host.example.com state.apply
host.example.com:
----------
----------
          ID: file_keyvalue_test
    Function: file.keyvalue
        Name: /tmp/test.txt
      Result: True
     Comment: Changed 1 lines
     Started: 22:46:00.444384
    Duration: 6.222 ms
     Changes:   
              ----------
              diff:
                  -   ServerName YOUR_SERVER_FQDN
                  + ServerName host.example.com

Summary for host.example.com
-------------
Succeeded: 1 (changed=1)
Failed:     0

4. View the file on minion:

# cat /tmp/test.txt 
<VirtualHost *:80>
ServerName host.example.com
  ServerSignature Off

  RewriteEngine on
  RewriteCond %{HTTPS} !=on
  RewriteRule .* https://%{SERVER_NAME}%{REQUEST_URI} [NE,R,L]
</VirtualHost>

Expected behavior
file.keyvalue should preserve indentation of key:value pairs:

# cat /tmp/test.txt 
<VirtualHost *:80>
  ServerName host.example.com
  ServerSignature Off

  RewriteEngine on
  RewriteCond %{HTTPS} !=on
  RewriteRule .* https://%{SERVER_NAME}%{REQUEST_URI} [NE,R,L]
</VirtualHost>

Versions Report

# salt --versions-report
Salt Version:
           Salt: 3001.1

Dependency Versions:
           cffi: 1.11.5
       cherrypy: unknown
       dateutil: 2.6.1
      docker-py: Not Installed
          gitdb: Not Installed
      gitpython: Not Installed
         Jinja2: 2.10.1
        libgit2: Not Installed
       M2Crypto: 0.35.2
           Mako: Not Installed
   msgpack-pure: Not Installed
 msgpack-python: 0.6.2
   mysql-python: Not Installed
      pycparser: 2.14
       pycrypto: Not Installed
   pycryptodome: Not Installed
         pygit2: Not Installed
         Python: 3.6.8 (default, Apr 16 2020, 01:36:27)
   python-gnupg: Not Installed
         PyYAML: 3.12
          PyZMQ: 19.0.0
          smmap: Not Installed
        timelib: Not Installed
        Tornado: 4.5.3
            ZMQ: 4.3.2

System Versions:
           dist: centos 8 Core
         locale: UTF-8
        machine: x86_64
        release: 4.18.0-193.19.1.el8_2.x86_64
         system: Linux
        version: CentOS Linux 8 Core

Start at the file.keyvalue state entry point and reproduce the issue using the /tmp/test.txt configuration and state shown in the report. Trace how the matching ServerName line is replaced, then add coverage for its two-space indentation. Done means applying the state changes the value while preserving the original indentation.