[5.5.x] Skip disk usage check if requirements not specified

[bernardjkim]

Description

This PR fixes a bug that was introduced during changes to the storage check due to satellite setting a default HighWatermark value to the storage checker. Storage checkers with unspecified or 0 HighWatermark value will skip the disk usage check.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• Regression fix (non-breaking change which fixes a regression)

Linked tickets and other PRs

• Refs Tracking back/forward ports #1740
• Requires [5.5.x] Remove watermark defaults satellite#238, [5.5.x] Set default watermark values in planet planet#694

TODOs

• Self-review the change
• Perform manual testing
• Address review feedback

Testing done

Verify successful install of single node cluster with mounted /proc volume

• Create mount for /proc node profile in telekube manifest

// Before changes, install fails due to failed pre-flight check
TASK [Install gravity application] *******************************************************************************************************************************************
task path: /home/bernard/go/src/github.com/gravitational/gravity/vagrant/ansible/install.yaml:8
fatal: [node-1]: FAILED! => {"changed": true, "cmd": "umask 0066\n cd /vagrant/installer\n  ./gravity install --debug --cloud-provider=generic --flavor=standalone --cluster=dev.test --advertise-addr=172.28.128.101 --token=token123\n ", "delta": "0:00:07.115842", "end": "2020-06-24 06:46:33.118211", "msg": "non-zero return code", "rc": 255, "start": "2020-06-24 06:46:26.002369", "stderr": "", "stderr_lines": [], "stdout": "Wed Jun 24 06:46:26 UTC\tStarting installer\nWed Jun 24 06:46:26 UTC\tPreparing for installation...\nWed Jun 24 06:46:33 UTC\tInstalling application telekube:5.5.48\nWed Jun 24 06:46:33 UTC\tStarting non-interactive install\n[ERROR]: The following pre-flight checks failed:\n\t[×] disk capacity at /proc is 0 (failed to validate storage requirements)", "stdout_lines": ["Wed Jun 24 06:46:26 UTC\tStarting installer", "Wed Jun 24 06:46:26 UTC\tPreparing for installation...", "Wed Jun 24 06:46:33 UTC\tInstalling application telekube:5.5.48", "Wed Jun 24 06:46:33 UTC\tStarting non-interactive install", "[ERROR]: The following pre-flight checks failed:", "\t[×] disk capacity at /proc is 0 (failed to validate storage requirements)"]}

// After changes, install is successful
TASK [Install gravity application] *******************************************************************************************************************************************
task path: /home/bernard/go/src/github.com/gravitational/gravity/vagrant/ansible/install.yaml:8
changed: [node-1] => {"changed": true, "cmd": "umask 0066\n cd /vagrant/installer\n  ./gravity install --debug --cloud-provider=generic --flavor=standalone --cluster=dev.test --advertise-addr=172.28.128.101 --token=token123\n ", "delta": "0:04:56.923701", "end": "2020-06-24 06:30:43.506789", "rc": 0, "start": "2020-06-24 06:25:46.583088", ...