Add allow_simult_ips_same_host field for targets

[timopollmeier]

What:
This adds the allow_simult_ips_same_host column to the targets and corresponding elements to the GMP commands create_target, get_targets and modify_target.
The value is sent to the scanner as a scanner preference.

Why:
This allows defining the scanner preference added in greenbone/openvas-scanner#604 as part of the target in case scanning multiple IP addresses of a single host causes problem

How:
I've checked the GMP responses of the get_targets command by inspecting the HTTP responses for the targets list and trashcan in GSA.

For the other two GMP commands I've used gvm-cli with GMP commands like the following with the values "0", "1" and "123":

<modify_target target_id="88c4b9e5-5b1c-4a0b-8b7c-b2b098676526">
  <allow_simult_ips_same_host>0</allow_simult_ips_same_host>
</modify_target>

<create_target>
  <allow_simult_ips_same_host>0</allow_simult_ips_same_host>
  <name>localhost test</name>
  <hosts>localhost</hosts>
  <port_list id="c7e03b6c-3bbe-11e1-a057-406186ea4fc5"/>
</create_target>

Checklist:

• Tests
• CHANGELOG Entry

[timopollmeier]

Note: The field has been renamed to allow_simultaneous_ips in #1396