Fix discover daemonset default value for discovery interval

[lbailleul]

Description of your changes:
Fix the default value passed to the discovery deamonset by the operator and add a configuration variable discoveryDaemonInterval to the Helm chart

Currently the daemonset pods crash with the error below, this is due to the operator not properly setting a default value when the ROOK_DISCOVER_DEVICES_INTERVAL env variable is not set.

Error: invalid argument "" for "--discover-interval" flag: time: invalid duration ""
   Usage:  rook discover [flags]

Flags:
rook error: invalid argument "" for "--discover-interval" flag: time: invalid duration ""
        --discover-interval duration   interval between discovering devices (default 60m) (default 1h0m0s)  
  -h, --help                                      help for discover      
        --use-ceph-volume                 Use ceph-volume inventory to extend storage devices information (default false)
Global Flags:      
       --log-level string   logging level for logging/tracing output (valid values: ERROR,WARNING,INFO,DEBUG) (default "INFO")

Which issue is resolved by this Pull Request:
Resolves #

Checklist:

• Commit Message Formatting: Commit titles and messages follow guidelines in the developer guide.
• [] Skip Tests for Docs: If this is only a documentation change, add the label skip-ci on the PR.
• Reviewed the developer guide on Submitting a Pull Request
• [] Pending release notes updated with breaking and/or notable changes for the next minor release.
• [] Documentation has been updated, if necessary.
• [] Unit tests have been added, if necessary.
• [] Integration tests have been added, if necessary.

[subhamkrai]

Please squash your commits. Thanks

[mergify]

This pull request has merge conflicts that must be resolved before it can be merged. @lbailleul please rebase it. https://rook.io/docs/rook/latest/Contributing/development-flow/#updating-your-fork