Add two new functions to the netconfig state

[mirceaulinic]

What does this PR do?

Both are very little and simply invoke existing Salt functions, but they are very handy (particularly the saved one has been demanded many times by the community).

SLS Example:

update_location:
  netconfig.replace_pattern:
    - pattern: Tampa
    - repl: Orlando

Produces the output:

$ sudo salt veos-lab state.apply test
veos-lab:
----------
          ID: update_location
    Function: netconfig.replace_pattern
      Result: True
     Comment: Configuration changed!
     Started: 17:50:12.012964
    Duration: 7909.62 ms
     Changes:
              ----------
              diff:
                  @@ -8,7 +8,7 @@
                   ntp server 172.17.17.2
                   ntp server 192.168.0.1
                   !
                  -snmp-server location Tampa, FL
                  +snmp-server location Orlando, FL
                   !
                   spanning-tree mode mstp
                   !

Summary for veos-lab
------------
Succeeded: 1 (changed=1)
Failed:    0
------------
Total states run:     1
Total run time:   7.910 s

And the other one - SLS Example:

/var/backups/routers/{{ opts.id }}/3rd_august_2018.cfg:
  netconfig.saved:
    - source: running
    - makedirs: true

Produces:

$ sudo salt veos-lab state.apply test
veos-lab:
----------
          ID: /var/backups/routers/veos-lab/3rd_august_2018.cfg
    Function: netconfig.saved
      Result: True
     Comment: File /var/backups/routers/veos-lab/3rd_august_2018.cfg updated
     Started: 17:52:49.708094
    Duration: 1511.962 ms
     Changes:
              ----------
              diff:
                  ---
                  +++
                  @@ -9,7 +9,7 @@
                   ntp server 172.17.17.2
                   ntp server 192.168.0.1
                   !
                  -snmp-server location Tampa, FL
                  +snmp-server location Orlando, FL
                   !
                   spanning-tree mode mstp
                   !

Summary for veos-lab
------------
Succeeded: 1 (changed=1)
Failed:    0
------------
Total states run:     1
Total run time:   1.512 s

[mirceaulinic]

All of these have been copied from states.file.managed. I wasn't sure if I should just point to that function, but given that some arguments are not relevant in this context, e.g., contents_pillar, contents_grains, etc., I thought it would be better to just copy the documentation here to avoid any confusion.