install-method-check.sh can fail if MAAS is specified by IP address

[Rod Smith]

The install-method-check.sh script runs a number of checks to identify the Ubuntu installation source. Among these is parsing the /var/lib/cloud/instance/datasource file, which normally looks something like this on a MAAS-deployed system:

DataSourceMAAS: DataSourceMAAS [http://10-193-36-0--23.maas-internal:5248/MAAS/metadata/]

Note that the MAAS server is identified by a rather awkward hostname (10-193-36-0--23.maas-internal); however, an IP address seems to sometimes be used instead:

DataSourceMAAS: DataSourceMAAS [http://10.193.37.117:5248/MAAS/metadata/]

When this happens, a subsequent check in verify_maas_ip() fails. This function uses the "host" command to convert the hostname into an IP address; but when it's already an IP address, the result is a failure of the test:

$ /usr/lib/plainbox-provider-checkbox/bin/install-method-check.sh
Installation data source is http://10.193.37.117:5248/MAAS/metadata/
ERROR: Unable to determine MAAS server's IP address
MAAS version is 1:2.9.2-9164-g.ac176b5c4-0ubuntu1~20.04.1
FAIL: System appears to have not been installed by MANIACS-compliant MAAS.

The verify_maas_ip() function should be made more robust against this condition.