Pass a string to mount_point_exists? method

[carbonin]

This method will return a false negative if passed a Pathname object, which is what we are doing currently.

example from an appliance:

irb(main):012:0> LinuxAdmin::LogicalVolume.mount_point_exists?(Pathname.new(ENV.fetch("APPLIANCE_PG_MOUNT_POINT")))
=> false
irb(main):013:0> LinuxAdmin::LogicalVolume.mount_point_exists?(Pathname.new(ENV.fetch("APPLIANCE_PG_MOUNT_POINT")).to_s)
=> true

Broken as of #277

Appliance builds which include this change will fail with the following output from journalctl -u appliance-initialize:

[root@localhost vmdb]# journalctl -u appliance-initialize
-- Logs begin at Mon 2017-10-16 08:53:57 EDT, end at Mon 2017-10-16 13:01:07 EDT. --
Oct 16 12:54:06 localhost.localdomain systemd[1]: Starting Initialize Appliance Database...
Oct 16 12:54:08 localhost.localdomain appliance-initialize.sh[1305]: TERM environment variable not set.
Oct 16 12:54:08 localhost.localdomain appliance-initialize.sh[1305]: create encryption key
Oct 16 12:54:08 localhost.localdomain appliance-initialize.sh[1305]: configuring internal database
Oct 16 12:54:08 localhost.localdomain appliance-initialize.sh[1305]: The disk for database must be a mount point
Oct 16 12:54:08 localhost.localdomain appliance-initialize.sh[1305]: Failed to configure internal database
Oct 16 12:54:08 localhost.localdomain systemctl[1576]: Removed symlink /etc/systemd/system/multi-user.target.wants/appliance-initialize.service.
Oct 16 12:54:08 localhost.localdomain systemd[1]: Started Initialize Appliance Database.

/cc @ailisp

[miq-bot]

Checked commit carbonin@eb7df40 with ruby 2.3.3, rubocop 0.47.1, and haml-lint 0.20.0
1 file checked, 0 offenses detected
Everything looks fine. 🍰

[Fryguy]

@carbonin I will merge as is, but we should also change LinuxAdmin to support both pathname and strings for the incoming parameter.