fix: use same COS_* partition sizes regardless of seaparate data disk (backport #968)

[mergify]

Problem:
When Harvester is installed with a single disk shared between OS and data, we set the COS_STATE parittion size to 15G. When using a separate data disk, we just use the Elemental default partition layout, which only gives COS_STATE of 8G. This causes problems with upgrades due to lack of space.

Solution:
This commit ensures we set a 15G COS_STATE parition in both of the above cases.

Related Issue:

• [BUG] Stuck upgrade from 1.4.0 to 1.4.1 harvester#7457
• [BUG] discrepancy in default OS partition sizes when using separate data disk harvester#7493

Test plan:

• Install Harvester with a shared OS/data disk. Check the COS_OEM, COS_RECOVERY and COS_STATE partition sizes:

  # lsblk -o NAME,LABEL,SIZE
  NAME    LABEL             SIZE
  [...]
  vda                       500G
  ├─vda1                      1M
  ├─vda2  COS_OEM            50M          <-- 50M is correct
  ├─vda3  COS_RECOVERY        8G          <-- 8G is correct
  ├─vda4  COS_STATE          15G          <-- 15G is correct
  ├─vda5  COS_PERSISTENT    150G
  └─vda6  HARV_LH_DEFAULT 326.9G
  [...]
  

• Install Harvester with a separate data disk. Check the COS_OEM, COS_RECOVERY and COS_STATE partition sizes. They should be the same as above:

  # lsblk -o NAME,LABEL,SIZE
  NAME   LABEL             SIZE
  [...]
  vda                      250G
  ├─vda1 COS_GRUB           64M
  ├─vda2 COS_OEM            50M          <-- 50M is correct
  ├─vda3 COS_RECOVERY        8G          <-- 8G is correct
  ├─vda4 COS_STATE          15G          <-- 15G is correct
  └─vda5 COS_PERSISTENT  226.9G
  vdb    HARV_LH_DEFAULT   128G
  

  (Ignore COS_GRUB -- in the examples above one node was installed with BIOS, the other with UEFI, sorry for any confusion)
• Prior to this fix, if you were to install Harvester with a separate data disk, COS_RECOVERY would only be 4G, and COS_STATE would be 8G.

  ---

  This is an automatic backport of pull request fix: use same COS_* partition sizes regardless of seaparate data disk #968 done by Mergify.