Initial impl for fgcp. Also additions to core for subnets

Example output:

d:\projects>curl --user "user:pws" http://localhost:3001/api/networks/UZXC0GRT-ZG8ZJCJ07-N?format=xml
```xml
<?xml version='1.0' encoding='utf-8' ?>
<network href='http://localhost:3001/api/networks/UZXC0GRT-ZG8ZJCJ07-N' id='UZXC0GRT-ZG8ZJCJ07-N'>
  <name>Network for Dies-DC-test</name>
  <state>STARTED</state>
  <address_block></address_block>
  <subnets>
    <subnet href='http://localhost:3001/api/subnets/UZXC0GRT-ZG8ZJCJ07-N-DMZ' id='UZXC0GRT-ZG8ZJCJ07-N-DMZ'></subnet>
    <subnet href='http://localhost:3001/api/subnets/UZXC0GRT-ZG8ZJCJ07-N-SECURE1' id='UZXC0GRT-ZG8ZJCJ07-N-SECURE1'></subnet>
  </subnets>
  <ports>
  </ports>
  <actions>
  </actions>
</network>
```

d:\projects>curl --user "user:pwd" http://localhost:3001/api/subnets?format=xml
```xml
<?xml version='1.0' encoding='utf-8' ?>
<subnets>
  <subnet href='http://localhost:3001/api/subnets/UZXC0GRT-ZG8ZJCJ07-N-DMZ' id='UZXC0GRT-ZG8ZJCJ07-N-DMZ'>
    <name>Dies-DC-test [DMZ]</name>
    <state>STARTED</state>
    <address_block>192.168.0.0/24</address_block>
    <type>PRIVATE</type>
    <network>UZXC0GRT-ZG8ZJCJ07-N</network>
    <ports>
    </ports>
    <actions>
    </actions>
  </subnet>
  <subnet href='http://localhost:3001/api/subnets/UZXC0GRT-ZG8ZJCJ07-N-SECURE1' id='UZXC0GRT-ZG8ZJCJ07-N-SECURE1'>
    <name>Dies-DC-test [SECURE1]</name>
    <state>STARTED</state>
    <address_block>192.168.1.0/24</address_block>
    <type>PRIVATE</type>
    <network>UZXC0GRT-ZG8ZJCJ07-N</network>
    <ports>
    </ports>
    <actions>
    </actions>
  </subnet>
</subnets>
```